Laser Welding Method, Laser-Welded Joint, Outside Sheathing Panel, and Body Structure for Rolling Stock

ABSTRACT

A side construction  302  has an outside sheathing  306  and outside sheathing reinforcement members  307 A and  307 B joined to the outside sheathing  306  interiorly of the outside sheathing  306 . The reinforcement member  307 A ( 307 B) is shaped like a hat in section. The reinforcement members  307 A provided in the vicinity of a window opening portion have a laser welding spacing L 1  of 80 mm and each have a hat width of 50 mm, while the reinforcement members  307 B provided on other part have a laser welding spacing L 1  of 100 mm and each have a hat width of 70 mm. The reinforcement members  307 A and  307 B each have a height of 25 mm.

TECHNICAL FIELD

The present invention relates to a laser welding method and a laserwelded joint, as well as an outside sheathing panel and a body structurefor rolling stock which are formed by using the method and the joint.The invention is particularly suitable for uncoated body structures forrolling stock or car bodies of buses, trucks and like cars.

BACKGROUND ART

Conventionally known body structures for rolling stock include astainless steel body structure (body structure made of stainless steelfor rolling stock), a light weight stainless steel body structure 201(outside sheathing+framework+outside sheathing reinforcement members) asshown in FIG. 45(a), a double-sheet stainless steel body structure 202(outside sheathing+integral press-molded inside sheathing) as shown inFIG. 45(b), and a double-skin type stainless steel body structure (seepatent document 1 for example). Such stainless steel body structureshave many advantages including freedom from coating, easy maintenance,and anticorrosion.

In joining the outside sheathing and the outside sheathing reinforcementmembers together to form a body structure for rolling stock, resistancespot welding is frequently used from the viewpoint of reduction inthermal strain. To avoid shunting electric current to previously weldedspots, the welding pitch is usually adjusted to about 50 to about 80 mm.

A body structure for general rolling stock, particularly, a sideconstruction has some points to which attention should be paid instrength design.

The “side construction”, as used here, means a structure comprising asingle or plural side outside sheathing panels (each having outsidesheathing and outside sheathing reinforcement members).

A side outside sheathing panel 101 is subjected mainly to an in-planeshear action by a vertical load F1 imposed by the car's own weight andpassengers, as shown in FIG. 46(a). Also, the side outside sheathingpanel is subjected to in-plane axial compressing and flexing actions bya load F2 imposed by forward and backward force exerted between adjacentcars (end compressing load) via a car coupler, as shown in FIG. 46(b). Abreaking mode to which attention should be paid first in strength designis buckling of the side outside sheathing panel, and a basic structureis determined based on the criteria of such buckling.

For example, in a portion where the outside sheathing is extensivelysubjected to a compressing action (for example, a lower portion of awainscot panel located centrally of a car under the end compressingload), outside sheathing reinforcement members 101 (stiffener) havingrequired antiplane stiffness are joined to the inside surface of outsidesheathing 102, as shown in FIG. 46(c). Since the side construction forrolling stock, in general, is subjected to a compressing action workingin the longitudinal direction of the car more intensively than in anyother direction, it is a common practice to position the outsidesheathing reinforcement members (stiffener) on the inside surface of theoutside sheathing to extend in the longitudinal direction of the car.

In a portion where the outside sheathing is extensively subjected mainlyto shearing (for example, a door pocket part immediately above a bogieunder vertical load), it is ideal that the outside sheathingreinforcement members are joined to the outside sheathing at an angle of45° with respect to the rail direction. However, since such angledjoining is complicated in the manufacture, the reinforcement members areactually positioned horizontally (in the rail direction) or vertically.These two positions are comparable to each other in terms of bucklingstrength.

However, such stainless steel body structures as mentioned above haveseveral problems.

(i) A first problem is a lowered strength against general buckling andlocal buckling.

Resistance spot welding is frequently used to join the outside sheathingand the outside sheathing reinforcement members together from theviewpoint of reduction in thermal strain, as described above. To avoidshunting electric current to previously welded spots, the welding pitchis usually adjusted to about 50 to about 80 mm. In this case, it ispossible that stress is not decentralized over each reinforcement memberas desired and, hence, a theoretical buckling strength cannot beobtained.

Specifically, the stiffener panel may have an antiplane flexuralstiffness lower than the theoretical value, which results in thepossibility of occurrence of general buckling caused by a load that islower than estimated. Also, the outside sheathing might buckle betweenadjacent welded spots by compression in a direction parallel with theoutside sheathing reinforcement members (stiffeners). Thus, the bucklingstrength of the outside sheathing against such local buckling is alsolower than the theoretical buckling strength.

As can be understood from the idea about buckling strength to bedescribed later, with compression stress exerted on the outsidesheathing in a direction parallel with outside sheathing reinforcementmembers (stiffeners) joined thereto at a pitch of 80 mm for example, theoutside sheathing can withstand as low as about 60 Mpa if thereinforcement members are spot-welded to the outside sheathing at 80 mmpitch, though the outside sheathing can withstand a stress up to 160 MPaif the reinforcement members are continuously joined thereto.

Further, initial strain occurs on the outside sheathing due to strainabout each spot caused by pressure contact. Such initial strain alsocauses local buckling strength to lower largely.

(ii) A second problem is permanent deformation in a stress concentratedportion (on the tensioned side) or local buckling (on the compressedside).

In the side outside sheathing, stress is concentrated at the corners ofan opening portion of the side outside sheathing. A side constructionfor commuter cars, in particular, has many openings such as windows anddoorways, and stress concentration at the corners of such an openingportion is problematic.

Such stress concentrated portions allow permanent deformation andbuckling to occur on the tensioned side and the compressed side,respectively, which will finally lead to failure. A conceivable remedyfor this problem is to increase the plate thickness on the tensionedside by providing the outside sheathing with an additional plate-shapedoutside sheathing reinforcement member interiorly, thereby relieving thestress. Theoretically, the same remedy is possible on the compressedside. However, the conventional stainless steel body structure assembledby resistance spot welding involves some problems.

That is, in resistance spot welding, the welding pitch is usuallyadjusted to about 50 to about 80 mm, as described above. In this case,it is possible that stress is not decentralized over the reinforcementplate as desired and, hence, a theoretical buckling strength is notobtained. Further, even though the reinforcement plate is added, thenumber of welded spots is increased for joining of the reinforcementplate and, as a result, initial strain occurs in the outside sheathingdue to strain about each spot caused by pressure contact and heating.The provision of such a reinforcement plate may cause local bucklingstrength to lower on the contrary.

(iii) A third problem is associated with watertightness.

Since resistance spot welding, which is frequently used in assembling astainless steel body structure, can form nothing but a lap joint,joining of an outside sheathing to another outside sheathing or anedging member (a window frame, door mask or the like) is achieved by lapjoint.

A contrivance to maintain the watertightness of such a joint is neededto prevent penetration of water from the outside. The watertightness ofthe lap joint is ensured by sandwiching a sealant between lap portionsin welding because the lap portions define very fine clearancestherebetween and because spot welding is an intermittent welding method.Alternatively, the watertightness is ensured by filling a sealant on lapend portions like a fillet.

However, it is possible that the seal is broken by aged deterioration ofthe sealant due to weather and washing to allow water to penetrate intothe car. Note that such a problem will not occur with body structures ofplain steel or aluminum alloy because such a body structure allowscontinuous welding to be used.

(vi) A fourth problem is associated with the aesthetic value of outsidesheathing (side outside sheathing and end outside sheathing).

Since resistance spot welding, which is frequently used in assembling astainless steel body structure, includes pressing in a spotted fashionduring welding, strain occurs around each of resulting welded spots dueto pressing force and heating, while impressions are formed on thewelded spots. The outside sheathing is aesthetically impaired by suchstain and impressions. Impairment of the aesthetic value of the sideoutside sheathing or end outside sheathing, in particular, will lowerthe product value.

Though “scorch” on the outside sheathing resulting from spot welding canbe eliminated by an electrolytic process, impressions are relativelydeep and hence cannot be rendered invisible by polishing or a likeprocess following the joining.

Though impressions can be covered with a color band (film), theimpressions can be more conspicuous at some view angle.

(v) A fifth problem is the complexity of the inside framework.

Conventionally, as a structure for mounting interior trim or equipmenton a body structure, screw seats are welded to the main structure or theinside framework, or fixtures are separately provided.

Such fixtures and screw seats are mostly designed individually for eachcar and a mounting place for such a fixture or screw seat differsdepending on the car type and the part.

Accordingly, the count of parts including screw seats, inside frameworkand fixtures increases and, hence, very much man-hour is required formaking and welding of such parts. In addition, dimensional control formounting of such parts is complicated because mounting places are notstandardized.

The inventors of the present invention have found that theaforementioned problems can be solved if laser welding is utilizedinstead of resistance spot welding in joining the outside sheathing andthe outside sheathing reinforcement members together.

In using laser welding as described above, a certain penetration depthis needed to suppress variations in laser-welded joint strength in orderto obtain a structure having stabilized quality. However, too large apenetration depth sometimes causes discoloration to occur on the reverseside opposite away from the welding side (back scorch) due to hightemperature oxidation or allow weld bead to be exposed. Methods ofsolving these problems are known (see patent document 2 for example).Alternatively, it is possible to obviate oxidative discoloration bycooling the reverse side opposite away from the welding side duringwelding or by post-treatment.

With the conventional techniques, however, a member forming a lap jointis slightly bent at its edge due to contraction of the welded portionthat has been heated locally by laser beam and such an edge bend mayappear as a ridge-like weld mark along a weld line on the reverse sideopposite away from the welded side. Such a ridge-like weld markresulting from the edge bend is not so serious as compared topenetration through of a molten pool or oxidative discoloration causedby laser beam. However, the user requests that such a weld mark beeliminated-in order to upgrade the quality and added value of theoutside sheathing.

As a remedy for such a weld mark, it is possible to perform joiningwhile controlling the laser power so as not to develop any weld mark.However, in cases where a 1.5 mm-thick lower plate and a 1.0 mm-thickupper plate are welded together, the bead width at the joint interfaceis 0.3 to 0.5 mm and the penetration depth into the lower plate is 0.1to 0.2 mm. With such a penetration depth, joining cannot be achieved atall under the influence of some disturbance. Conversely, development ofa ridge-like weld mark proves that the joint has been formed certainly.It is therefore difficult to ensure both the joint quality and theaesthetic quality of the outward appearance at a time.

In the case of a lower plate having a thickness of not less than 3 mm,it has been confirmed that little influence is exerted on the outsidesheathing surface even if a sufficient penetration depth is ensured.However, an increase in the plate thickness of the lower plate more thannecessary results in a considerable increase in the mass of theresulting structure undesirably.

As a result of repeated intensive study made by the inventors, it hasbeen found that, if the outside surface of the outside sheathing issubjected to polishing (for example, belt grinder finishing generallyemployed for stainless steel body structures for rolling stock)substantially in parallel with the aforementioned weld line formed bylaser beam, the weld line is rendered substantially invisible byscattering of light.

Patent document 1: Japanese Patent Publication No. 2763983

Patent document 2: Japanese Patent Publication No. 2929447

DISCLOSURE OF INVENTION Problems to be Solved by Invention

The present invention has been made in view of the above-describedcircumstances. It is an object of the present invention to provide alaser welding method capable of rendering a weld mark visuallyinconspicuous, a laser-welded joint, an outside sheathing panel, and abody structure for rolling stock, as well as a body structure forrolling stock which is free from a decrease in buckling strength due tobuckling between adjacent welded spots or initial strain and hence canenjoy a required buckling strength.

MEANS FOR SOLVING PROBLEMS

A welding method according to the present invention is a laser weldingmethod comprising the steps of lapping plural plate members one overanother; continuously irradiating one of the lapped plate members withmoving laser beam from an antiplane direction to heat the lapped platemembers until a region extending to an inner portion of another one ofthe lapped plate members which is positioned on an opposite side awayfrom the plate member irradiated with the laser beam becomes molten; andjoining the plural plate members together while controlling the laserpower or moving speed of the laser beam so as not to allow a bottomportion of a resulting molten pool to reach an outside surface of theplate member positioned on the opposite side, the laser welding methodcharacterized by previously polishing the outside surface of theopposite-side plate member substantially in parallel with the directionof a weld line to be formed by the laser beam. The “plate member”, asused herein, need not necessarily be wholly plate-shaped but is meant toinclude a member having a plate-shaped portion to be subjected tojoining by laser welding. The “polishing”, which is also called BG (beltgrinder) finishing by means of a belt grinder, is a process forfinishing a steel sheet surface by grinding the steel sheet surfaceintermittently (discontinuously) in one direction by means of arevolving belt attached with abrasive grains conforming to theprescription “JIS R 6001”. (The BG finishing is surface finishingfalling under the category No. 3 or No. 4 of “JIS G 4305”.)

This method uses a plate member that has been polished substantially inparallel with the direction of a weld line to be formed by laser beam asa plate member forming an outside surface. Such a plate member rendersthe weld line substantially invisible by scattering of light and hencerenders a ridge-like weld mark resulting on the outside surface visuallyinconspicuous without the need to perform surface finishing on theoutside surface as a post-treatment.

A welded joint according to the present invention is a laser-weldedjoint comprising plural plate members joined together by: lapping theplural plate members one over another; continuously irradiating one ofthe lapped plate members with moving laser beam from an antiplanedirection to heat the lapped plate members until a region extending toan inner portion of another one of the plate member which is positionedon an opposite side away from the plate member irradiated with the laserbeam becomes molten; and joining the plural plate members together whilecontrolling the laser power or moving speed of the laser beam so as notto allow a bottom portion of a resulting molten pool to reach an outsidesurface of the plate member positioned on the opposite side, thelaser-welded joint characterized in that the outside surface of theopposite-side plate member is polished substantially in parallel withthe direction of a weld line formed by the laser beam. The “beingpolished substantially in parallel with the direction of a weld lineformed by the laser beam”, as expressed herein, is meant to include thefollowing cases (i) to (iii) which occur when the weld line directioncannot be limited to one specific direction in view of the structure ofa plate member (fitting part such as framework material or liningmaterial) to be joined to the inside surface of the plate member formingthe outside surface of the joint. That is, case (i): the polishingdirection is made to coincide with the direction of a weld line that ismost influential on the outward appearance (outside surface); case (ii):a plate member that will define a weld line not coinciding with thepolishing direction is welded to a plate member joined along a weld linesubstantially coinciding with the polishing line; and case (iii) aprotective or surfacing material is mounted to cover a weld markextending on a weld line not coinciding with the polishing direction (aridge-shaped weld mark caused by edge bend) in order for the weld markto be visually unobservable or difficult to observe from the outsidesurface side. The polishing of the outside surface of the plate membersubstantially in parallel with the direction of the weld line formed bylaser beam may be conducted either before or after the joining.

Since the plate member forming the outside surface is thus subjected topolishing substantially in parallel with the direction of the weld lineto be formed by laser beam, scattering of light on the outside surfacediscontinuously polished renders the weld mark along the weld linesubstantially invisible. That is, the ridge-like weld mark developed onthe outside surface by edge bend becomes visually inconspicuous, thusresulting in an improvement in the aesthetic value of the outsidesheathing.

By thus using a member polished substantially in parallel with the weldline by means of a belt grinder or the like as a component of a weldedlap joint which forms the outside surface (the lower side of the lowerplate member forming the welded lap joint) on the reverse side oppositeaway from the welding side, the weld mark developed as described aboveon even on a partial penetration welded joint having sufficientpenetration can hardly be observed visually (can be rendered visuallyinconspicuous) by scattering of light or the like. Thus, the outsidesurface can be made aesthetically satisfactory. That is, it becomespossible to ensure both a stabilized and appropriate joint strength andan outward appearance of aesthetic quality.

Description will be made of a case where two plate members arelap-joined together as a specific example of the above-describedarrangement. Referring first to FIG. 1(a), two plate members 1 and 2(for example, cold-rolled stainless steel sheets such as SUS301L orSUS304) are lapped one over the other; the lapped plate members 1 and 2are continuously irradiated with moving laser beam R from the side ofthe plate member 2 opposite away from the side contacting the platemember 1 so as to be heated until a region extending to an inner portionof the plate member 1 opposed to the plate member 2 irradiated with thelaser beam R becomes molten; and the plate members 1 and 2 are joinedtogether, while the laser power or moving speed of the laser beam R iscontrolled so as not to allow a bottom portion of a resulting moltenpool to reach an outside surface S1 of the plate member 1 on theopposite side. No weld bead appears on the outside surface S1. In thiscase, a ridge-like weld mark is developed by a slight edge bend by anangle θ caused by heat shrinkage following laser welding, as shown inFIG. 1(b). This edge bend (edge bend line L) appears as a streak on theoutside surface of the outside plate member 1. Nevertheless, the edgebend (edge bend line L) as a weld mark is rendered substantiallyinvisible by scattering of light incident on the outside surface S1 asshown in FIG. 1(d) because the outside surface S1 is polished inparallel with the weld line. If the surface is smooth, light rays from aspecific direction are definitely divided by the edge bend in directionsR1 and R2 of reflection toward a point of view, as shown in FIG. 17(a).For this reason, the edge bend line L can be observed. In contrast, ifthe surface is polished in the weld line direction, light rays from alldirections are reflected toward the point of view as shown in FIG.17(b), thus making the edge bend L invisible.

Thus, (i) there is an advantage in improving the product quality byvirtue of the aesthetically excellent outside surface with the weld linerendered invisible and improved dimensional accuracy with less weldingstrain. (ii) Since laser welding exhibits an increased welding speed,hence, an improved workability, the manufacturing cost can be reduced.(iii) Since laser welding is capable of continuous welding, any sealantcan be eliminated, which leads to a reduction in the maintenance cost.

An outside sheathing panel according to the present invention comprisesan outside sheathing, and a first reinforcement member lapped over theouter sheathing interiorly of the outside sheathing, the outer sheathingand the first reinforcement member being joined together by beingcontinuously irradiated with moving laser beam from the firstreinforcement member side so as to be heated until a region extending toan inner portion of the outside sheathing becomes molten with the laserpower or moving speed of the laser beam controlled so as not to allow abottom portion of a resulting molten pool to reach an outside surface ofthe outside sheathing, the outside sheathing panel characterized in thatthe outside surface of the outside sheathing is polished substantiallyin parallel with the direction of a weld line formed by the laser beam.

Since the outside surface of the outside sheathing is polishedsubstantially in parallel with the direction of the weld line formed bythe laser beam, scattering of light on the outside surface renders theweld line on the outside surface substantially invisible. That is, aridge-like weld mark developed on the outside surface of the outsidesheathing panel (outside sheathing) becomes visually inconspicuous.

The above-described outside sheathing panel can be used in a bodystructure for rolling stock. That is, a body structure for rolling stockaccording to the present invention comprises an outside sheathing panelas a side outside sheathing of a side construction, the outsidesheathing panel comprising an outside sheathing, and a firstreinforcement member lapped over the outer sheathing interiorly of theoutside sheathing, the outer sheathing and the first reinforcementmember being joined together by being continuously irradiated withmoving laser beam from the first reinforcement member side so as to beheated until a region extending to an inner portion of the outsidesheathing becomes molten with the laser power or moving speed of thelaser beam controlled so as not to allow a bottom portion of a resultingmolten pool to reach an outside surface of the outside sheathing, thebody structure characterized in that: the outside surface of the outsidesheathing of the outside sheathing panel is polished substantially inparallel with the direction of a weld line formed by the laser beam; andthe direction of the weld line coincides with a specific direction(longitudinal or heightwise direction of the car).

In the body structure for rolling stock thus constructed, since theoutside sheathing of the outside sheathing panel is polishedsubstantially in parallel with the direction of the weld line formed bythe laser beam, scattering of light on the outside surface renders theweld line substantially invisible. Thus, a ridge-like weld markdeveloped on the outside surface becomes visually inconspicuous, whichresults in an improvement in the aesthetic value of the outwardappearance of the outside sheathing. Such a body structure isparticularly suitable for an uncoated body structure for rolling stock.

In the body structure for rolling stock according to the presentinvention, it is desirable that: the first reinforcement member have aportion shaped like a hat in section; and that portion has a body partsubstantially U-shaped in section and flange parts extendingcontinuously from opening defining edges of the body part in oppositedirections and joined to the outside sheathing by laser welding, thebody part extending in a longitudinal direction of a railway car.

With such a feature, the first reinforcement member and the outsidesheathing form a completely closed section, which imparts the firstreinforcement member with a very high flexural stiffness in thelongitudinal direction of the first reinforcement member while enhancingthe buckling strength of the outside sheathing in that direction. Also,the weld line direction and the polishing direction can be madecoinciding with each other easily because the first reinforcement memberis joined to the outside sheathing so as extend in the longitudinaldirection of the railway car.

The substantially U-shaped section of the body part may define therein aspace having a constant width from the bottom to the open side.Alternatively, it is possible that: the substantially U-shaped sectionof the body part defines therein a space gradually widening as the spaceextends toward the open side; the first reinforcement member is providedon the opposite side away from the outside sheathing with a secondreinforcement member which is shaped like a hat in section and extendsin a direction perpendicular to the first reinforcement member; and thesecond reinforcement member has flange parts joined to the body part ofthe first reinforcement member. Here, though there is no particularlimitation on the shape of the second reinforcement member, the secondreinforcement member is desirably shaped like a hat in section so as toensure an adequate weld length. Also, though there is no particularlimitation on the method of joining the second reinforcement member, itis possible that joining is made by laser welding so that the bottomportion of a molten pool penetrates through the body part of the firstreinforcement member.

With such a feature, the second reinforcement member having a hat-shapedsection is lapped over the first reinforcement member so as to extend ina direction perpendicular to the first reinforcement member and, hence,the antiplane flexural stiffiness in the direction perpendicular to thefirst reinforcement member can be enhanced. Also, since the firstreinforcement member joined to the outside sheathing by laser welding isshaped like a hat (trapezoidal) in section, the stiffness againstantiplane shear deformation in the direction perpendicular to thelongitudinal direction of the first reinforcement member can beenhanced. Further, since laser welding is conducted so that the bottomportion of the resulting molten pool penetrates through the body part ofthe first reinforcement member, a sufficient joint strength can bestably ensured without the outward appearance affected.

Another body structure for rolling stock according to the presentinvention comprises an outside sheathing of a side construction, andthird reinforcement members joined to the outside sheathing interiorlyof the outside sheathing by laser welding for stiffening the outsidesheathing, the third reinforcement members being arranged in alongitudinal direction of a railway car, the body structurecharacterized in that the third reinforcement members are arranged at apitch defined by spacing between adjacent weld lines formed by the laserwelding, the pitch being established such that uniaxial stresses ox andoy in respective of the longitudinal direction of the railway car and adirection perpendicular thereto and a shear stress τ, which are obtainedby resolution of an in-plane stress component exerted on the outsidesheathing, are not larger than a buckling stress value determined byEuler's buckling formula and a plasticity correction formula. Though thelaser welding of the third reinforcement members to the outsidesheathing is continuous welding basically, continuous welding may not beconducted on a portion which does not need continuous welding for astructural reason (for example, a portion which is unlikely to bucklebecause of a low stress to be exerted on the outside sheathing) or on aportion which does not allow continuous welding to be conducted thereonfor a structural reason.

The above-described invention has been made based on the following ideaabout buckling strength.

The Euler's buckling formula for a rectangular flat plate is expressedas the following formula (see FIG. 18(a)): $\begin{matrix}\left\lbrack {{Numerical}\quad{formula}\quad 1} \right\rbrack & \quad \\{\sigma_{cr}^{e} = {K\frac{\quad{\pi^{2}E}}{12\left( {1 - v^{2}} \right)}\left( \frac{t}{b} \right)^{2}}} & (1)\end{matrix}$

Here, E represents a modulus of longitudinal elasticity, v represents aPoisson's ratio and t represents the thickness of the flat plate. Krepresents a buckling factor. When the rectangular plate is simplysupported (rotational support) by its four sides, K is theoreticallyrepresented as: $\begin{matrix}\left\lbrack {{Numerical}\quad{formula}\quad 2} \right\rbrack & \quad \\{K = \left\{ \begin{matrix}\left( {\alpha + \frac{1}{\alpha}} \right)^{2} & \left( {\alpha < 1} \right) \\4 & \left( {\alpha \geq 1} \right)\end{matrix} \right.} & (2)\end{matrix}$

Here, α represents a side length ratio expressed as: $\begin{matrix}\left\lbrack {{Numerical}\quad{formula}\quad 3} \right\rbrack & \quad \\{\alpha = \frac{a}{b}} & (3)\end{matrix}$

When the stress produced is large, the influence of plasticity is notnegligible. In such a case, plasticity correction is made using, forexample, Johnson's experimental formula as follows:

[Numerical Formula 4] $\begin{matrix}{\sigma_{cr} = \left\{ \begin{matrix}\sigma_{cr}^{e} & \left( {\sigma_{cr}^{e} \leq {\sigma_{Y}/2}} \right) \\{\sigma_{Y}\left( {1 - \frac{\sigma_{Y}}{4\sigma_{cr}^{e}}} \right)} & \left( {\sigma_{cr}^{e} > {\sigma_{Y}/2}} \right)\end{matrix} \right.} & (4)\end{matrix}$

Here, σY represents the yield strength of the material.

Similarly, the shear buckling stress is expressed as follows.$\begin{matrix}\left\lbrack {{Numerical}\quad{formula}\quad 5} \right\rbrack & \quad \\{\tau_{cr}^{e} = {K\quad\frac{\pi^{2}E}{12\left( {1 - v^{2}} \right)}\left( \frac{t}{b} \right)^{2}}} & (5) \\{K = {6\quad\left( {{{four}\quad{side}\quad{support}},{\alpha ⪢ 1}} \right)}} & (6) \\{\tau_{cr} = \left\{ \begin{matrix}\tau_{cr}^{e} & \left( {{\tau_{cr}^{e} \leq {\tau_{Y}/2}} = {{\sigma_{Y}/2}\sqrt{3}}} \right) \\{\tau_{Y}\left( {1 - \frac{\tau_{Y}}{4\tau_{cr}^{e}}} \right)} & \left( {{\tau_{cr}^{e} > {\tau_{Y}/2}} = {{\sigma_{Y}/2}\sqrt{3}}} \right)\end{matrix} \right.} & (7)\end{matrix}$

(See FIG. 18(b).)

In an actual body structure, an outside sheathing panel is constructedby joining reinforcement members to a thin-sheet outside sheathing for astiffening purpose. The arrangement and the pitch of the reinforcementmembers have to be determined so as not to allow a rectangular region ofthe outside sheathing surrounded by the reinforcement members to buckle.

With respect to the in-plane stress exerted on the outside sheathing ofthe side construction by either of a vertical load or an end compressingload imposed on the body structure, the stress exerted in thelongitudinal direction of the car is larger than that exerted in thedirection perpendicular to the longitudinal direction (in the verticaldirection) at many points. For this reason, the reinforcement membersshould be arranged so that the long sides of the rectangular region ofthe outside sheathing to be surrounded by the reinforcement membersextend coincidentally with the longitudinal direction of the car. Thisis because the side construction has a buckling strength againstcompression along the long side several times as high as a bucklingstrength against compression along the short side, as can be seen fromthe buckling mode shown in FIG. 18(c).

This can be expressed by the following numerical formulae:$\begin{matrix}\left\lbrack {{Numerical}\quad{formula}\quad 6} \right\rbrack & \quad \\{\sigma_{{cr},x}^{e} = {{4 \times \frac{\pi^{2}E}{12\left( {1 - v^{2}} \right)}\left( \frac{t}{b} \right)^{2}} = {4 \times {A\left( \frac{t}{b} \right)}^{2}\left( {\alpha ⪢ 1} \right)}}} & (8) \\\begin{matrix}{\sigma_{{cr},y}^{e} = {{\left( {\alpha + \frac{1}{\alpha}} \right)^{2} \times \frac{\pi^{2}E}{12\left( {1 - v^{2}} \right)}\left( \frac{t}{a} \right)^{2}} = {\frac{1}{\alpha^{2}}\left( {\alpha + \frac{1}{\alpha}} \right)^{2} \times {A\left( \frac{t}{b} \right)}^{2}}}} \\{= \left. {\left( {1 + \frac{2}{\alpha^{2}} + \frac{1}{\alpha^{4}}} \right) \times {A\left( \frac{t}{b} \right)}^{2}}\rightarrow{{A\left( \frac{t}{b} \right)}^{2}\left( \alpha\rightarrow\infty \right)} \right.}\end{matrix} & (9) \\{\tau_{cr}^{e} = {{6 \times \frac{\pi^{2}E}{12\left( {1 - v^{2}} \right)}\left( \frac{t}{b} \right)^{2}} = {6 \times {A\left( \frac{t}{b} \right)}^{2}\left( {\alpha ⪢ 1} \right)}}} & (10) \\{{wherein},{A = \frac{\pi^{2}E}{12\left( {1 - v^{2}} \right)}}} & \quad\end{matrix}$

Therefore, the approximation made by the above formulae holds well ifthe length of the long side is five times or more as large as that ofthe short side.

FIGS. 19(a) to 19(c) plot the relationships between the short sidelength b of the rectangular region and buckling stresses (σcrx, σcr,yand τcr).

The pitch at which the reinforcement members are arranged on the outsidesheathing panel is considered to be equal to the short side length b ofthe rectangular region.

Conversely, if the stresses (σx, σy and τ) to be produced can beestimated through stress analysis or structural test, the pitch b atwhich the reinforcement members are to be arranged can be determinedfrom FIGS. 19(a) to 19(C). The pitch b can be expressed by the followingformula: $\begin{matrix}\left\lbrack {{Numerical}\quad{formula}\quad 7} \right\rbrack & \quad \\{{b = {\min\left( {{t\sqrt{\frac{4A}{\sigma_{x}^{*}}}},{t\sqrt{\frac{A}{\sigma_{y}^{*}}}},{t\sqrt{\frac{6A}{\tau^{*}}}}} \right)}}{wherein}} & (11) \\\left\lbrack {{Numerical}\quad{formula}\quad 8} \right\rbrack & \quad \\{\sigma_{x}^{*} = \left\{ \begin{matrix}\sigma_{x} & \left( {\sigma_{x} \leq {\sigma_{Y}/2}} \right) \\{\sigma_{Y}^{2}/\left\{ {4\left( {\sigma_{Y} - \sigma_{x}} \right)} \right\}} & \left( {\sigma_{x} > {\sigma_{Y}/2}} \right)\end{matrix} \right.} & (12) \\{\sigma_{y}^{*} = \left\{ \begin{matrix}\sigma_{y} & \left( {\sigma_{y} \leq {\sigma_{Y}/2}} \right) \\{\sigma_{Y}^{2}/\left\{ {4\left( {\sigma_{Y} - \sigma_{y}} \right)} \right\}} & \left( {\sigma_{y} > {\sigma_{Y}/2}} \right)\end{matrix} \right.} & (13) \\{\tau^{*} = \left\{ \begin{matrix}\tau & \left( {\tau \leq {{\sigma_{Y}/2}\sqrt{3}}} \right) \\{\sigma_{Y}^{2}/\left\{ {12\left( {{\sigma_{Y}/\sqrt{3}} - \tau} \right)} \right\}} & \left( {\tau > {{\sigma_{Y}/2}\sqrt{3}}} \right)\end{matrix} \right.} & (14)\end{matrix}$

Yet another body structure for rolling stock according to the presentinvention comprises an outside sheathing of a side construction, andthird reinforcement members joined to the outside sheathing interiorlyof the outside sheathing by laser welding for stiffening the outsidesheathing, the third reinforcement members being arranged in a raildirection, the body structure characterized in that the thirdreinforcement members are arranged at a pitch defined by spacing betweenadjacent weld lines formed by the laser welding, the pitch beingestablished based on a buckling strength of a portion at which a load tobe imposed on the third reinforcement members becomes maximized in adirection in which the third reinforcement members extends.

This construction, which employs continuous laser welding as joiningmeans to join the outside sheathing and the third reinforcement memberstogether, is free from a decrease in buckling strength due to bucklingbetween adjacent welded spots or initial strain which is essential tojoining by the conventional resistance spot welding and hence can obtaina required buckling strength. Accordingly, if the pitch of thereinforcement members is determined theoretically, there is no need toprovide the outside sheathing with additional small reinforcement platesfor further reinforcement, which can contributes to a reduction in partscount and in man-hour.

Yet another body structure for rolling stock according to the presentinvention comprises an outside sheathing of a side construction, andthird reinforcement members joined to the outside sheathing interiorlyof the outside sheathing by laser welding for stiffening the outsidesheathing, the third reinforcement members being arranged in alongitudinal direction of a railway car, wherein it is possible that:the third reinforcement members are each shaped like a hat in sectionand each have a U-shaped part substantially U-shaped in section and twofitting flange parts extending continuously from end edges of theU-shaped part in opposite directions and joined to the outside sheathingby laser welding; and the U-shaped part of each of the thirdreinforcement members is smaller in width than a U-shaped part of afirst member provided on other part of the body structure. This bodystructure is characterized by stringers separately provided to stiffenthe body structure against antiplane flexure in a directionperpendicular to the longitudinal direction of the railway car.

This construction uses the third reinforcement members having two typesof sectional shape, the number of which is markedly smaller thanrequired by the conventional art, thus leading to a reduction in theparts manufacturing cost. Also, the decreased number of types of partsmakes it easy to improve the dimensional accuracy of the parts.

In the present invention, it is possible that: the third reinforcementmembers are each shaped like a hat in section and each have a U-shapedpart substantially U-shaped in section and two fitting flange partsextending continuously from end edges of the U-shaped part in oppositedirections and joined to the outside sheathing by laser welding; and afourth reinforcement member is previously mounted in the U-shaped partof each of the third reinforcement members for enhancing the stiffnessof a portion of the outside sheathing around the U-shaped part.

With such a feature, even when an insufficient local buckling strengthresults despite the provision of the third reinforcement members,necessary and minimum reinforcement can be provided reliably and easilywithout increase in man-hour.

In the present invention, each of the third reinforcement members mayhave a head portion having an opening or a notch for allowing the fourthreinforcement member to be joined to the outside sheathing by laserwelding.

This feature allows the fourth reinforcement members as well as thethird reinforcement members to be joined directly to the outsidesheathing. Thus, it is possible to enhance local reinforcement of theoutside sheathing thereby to enhance the buckling strength efficiently.

In the present invention, each of the third reinforcement members mayhave a root portion joined to the outside sheathing by laser welding,and a hollow bulged portion having closed ends.

With such a feature, each of the third reinforcement members has no freeedge ends and hence enjoys an enhanced buckling strength and stiffnessat its end portions.

In the present invention, it is desirable that an inside plate frame isjoined to end portions of those third reinforcement members locatedadjacent a corner of an opening portion formed in the outside sheathingby laser welding to interconnect the end portions.

With such a feature, such a simple arrangement as to provide the insideplate frame makes it possible to reinforce the end portions of the thirdreinforcement members easily, thereby to enhance the strength of thecorner portion of the opening portion on which concentration of stressis likely.

In the present invention, it is possible that the third reinforcementmembers and the inside plate frame are joined together by laser weldingso as to form closed weld lines. The “closed weld line” is meant toinclude a ring-shaped (elliptical or circular) weld line and arectangular weld line.

With such a feature, even though the joint portion to be made by laserwelding is narrow and small, a long weld line can be ensured. Further,since the weld line is closed, concentration of stress on the weld beadends will not occur, which can ensure a required strength.

In the present invention, it is desirable that: the outside sheathinghas an opening portion having a peripheral edge provided with an edgingmember; and the edging member is formed with a notch for allowing ahold-down roller to advance thereinto in laser-welding the thirdreinforcement members to the outside sheathing, the hold-down rollerbeing configured to press a portion of each of the third reinforcementmembers to be joined against the outside sheathing while moving togetherwith the laser beam.

With such a feature, it is possible to avoid interference between theedging member and the hold-down roller at a location adjacent the endportion (edging member) of each third reinforcement member, thereby toallow continuous laser welding up to a point adjacent the end portion tobe achieved.

In the present invention, it is possible that: the third reinforcementmembers are each shaped like a hat in section and each have a U-shapedpart substantially U-shaped in section and two fitting flange partsextending continuously from end edges of the U-shaped part in oppositedirections and joined to the outside sheathing by laser welding; aninside plate frame is joined to free edge ends of the thirdreinforcement members outwardly of the free edge ends; and the insideplate frame has a body part joined to the U-shaped parts of the thirdreinforcement members, a leg part extending continuously from the bodypart in such a direction as to close the free edge ends, and a flangepart extending continuously from the leg part and joined to the outsidesheathing.

With such a feature, the end portions of the third reinforcement membersare joined to the outside sheathing via the inside plate frame and,hence, the stiffness and strength of the end portions can be enhanced.

In the present invention, it is possible that: an inside plate frame isjoined to free edge ends of the third reinforcement members outwardly ofthe free edge ends; and the inside plate frame has a first flange partjoined to the U-shaped parts of the third reinforcement members, asecond flange part joined to the outside sheathing, first and second legparts extending parallel from respective of the first and second flangeparts to a same height, and a horizontal part extending parallel withthe outside sheathing to interconnect upper end edges of respective ofthe first and second leg parts.

With such a feature, the inside plate frame has a larger height than thethird reinforcement member and hence can enhance the antiplanestiffness.

In the present invention, it is possible that at least one of a sideoutside sheathing of the side construction and a roof outside sheathingof a roof construction has an end edge portion extending in thelongitudinal direction of the railway car and having a folded part bywhich the side outside sheathing and the roof outside sheathing arejoined together by continuous laser welding.

With such a feature, the provision of the folded part can enhance thestiffness and, hence, the folded part can serve as a counterforcereceiver during laser welding.

In the present invention, it is desirable that the folded part has aU-shaped section which opens upwardly of the roof outside sheathing.

This feature can ensure required stiffness while forming a rainwaterguttering (U-shaped part).

In the present invention, it is desirable that: an end outside sheathingof an end construction and a side outside sheathing of a sideconstruction be joined together perpendicularly to each other via an endcorner post having a substantially L-shaped section; the end corner postbe either joined with a separate member defining a closed spacecooperatively with the end corner post by continuous laser welding orformed with a closed space forming part defining the closed spacecooperatively with the end corner post; and the closed space beconfigured to function as a rainwater guttering for guiding rainwaterfrom a roof to the ground.

This feature can realize the rainwater guttering for guiding rainwaterfrom the roof to the ground without the need to provide any additionalmember such as a water tube.

In the present invention, the outside sheathing desirably has an outsidesurface which is surface-finished by brushing or shot blasting.

With such a feature, a stainless steel body structure with its outsidesheathing having a high-precision outside surface of aesthetic value.

In the present invention, it is possible that: the outside sheathing isinteriorly fitted with a continuous rail member which is U-shaped insection for mounting interior trim or equipment on the outside sheathingby means of a mounting bolt having a head portion configured to engagethe rail member.

With such a feature that the outside sheathing is fitted with the railmember, it is possible to reduce the parts count and the man-hour formounting secondary structural material such as inside framework andfacilitate dimensional control.

In the present invention, the outside sheathing is desirably thickerthan each of the third reinforcement members and has a plate thicknessof not less than 3 mm.

With such a feature, no weld line appears on the surface of the outsidesheathing.

In the present invention, it is possible that: the outside sheathingdefines an opening portion having a peripheral edge exteriorly providedwith an edging member; and the edging member has a joint portion joinedto the outside sheathing by continuous laser welding from the outsidesheathing side, the joint portion comprising a folded plate structureformed by folding a flat plate.

With such a feature, no weld line appears on the surface of the outsidesheathing even if piercing welding is conducted.

ADVANTAGE OF INVENTION

With the welding method, welded joint, outside sheathing panel and bodystructure for rolling stock according to the present invention, the bodystructure stiffness (weight reduction) and the dimensional accuracy aswell as the outward appearance can be improved as compared to aconventional joint formed by resistance spot welding. Also, with theoutside sheathing having an outside surface polished substantially inparallel with the direction of a weld line formed by laser beam, nopost-treatment is required and, hence, the productivity can also beimproved.

Another body structure for rolling stock according to the presentinvention is free from a decrease in buckling strength due to bucklingbetween adjacent welded spots or initial strain by virtue of continuouslaser welding used to join the third reinforcement members to theoutside sheathing of the side construction for stiffening the outsidesheathing and hence can enjoy a required buckling strength. Accordingly,if the pitch of the reinforcement members is determined theoretically,there is no need to provide the outside sheathing with additional smallreinforcement plates for further reinforcement, which can contributes toreduction in parts count and in man-hour.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(a) to 1(d) are each an explanatory view illustrating theprinciple underlying a laser-welded joint according to the presentinvention.

FIGS. 2(a) to 2(c) are each an explanatory view illustrating an outsidesheathing panel according to the present invention.

FIG. 3 is a view showing a side construction for rolling stock accordingto the present invention as viewed from the outside of the railway car.

FIG. 4 is a view showing the side construction for rolling stock asviewed from the inside of the railway car.

FIG. 5 is a sectional view taken along line A-A of FIG. 4.

FIG. 6 is a sectional view taken along line B-B of FIG. 4.

FIG. 7 is a sectional view taken along line C-C of FIG. 4.

FIG. 8 is a sectional view taken along line D-D of FIG. 4.

FIG. 9 is a sectional view taken along line E-E of FIG. 4.

FIG. 10 is a sectional view taken along line F-F of FIG. 4.

FIG. 11 is an enlarged view showing a portion G shown in FIG. 5.

FIG. 12 is an enlarged view showing a portion H shown in FIG. 7.

FIG. 13 is an enlarged view showing a portion J shown in FIG. 7.

FIG. 14 is an enlarged view showing a portion K shown in FIG. 7.

FIG. 15 is a diagram showing the relationship between the width of ajoint portion of upper and lower plates and the tensile shear strengthper unit weld length.

FIG. 16 is an explanatory view illustrating one exemplary procedure forassembling a side construction for rolling stock according to thepresent invention.

FIGS. 17(a) and 17(b) are each an explanatory view illustrating theprinciple based on which a weld line is rendered invisible.

FIG. 18(a) is an explanatory view illustrating a buckling problemassociated with a rectangular plate; FIG. 18(b) is an explanatory viewillustrating a shear buckling problem associated with a rectangularplate; and FIG. 18(a) is an explanatory view illustrating a bucklingmode of an elongate rectangular plate.

FIG. 19(a) is an explanatory diagram plotting the relationship betweenshort side length b and buckling stress σcrx along the short side; FIG.19(b) is an explanatory diagram plotting the relationship between shortside length b and buckling stress σcr,y along the short side; and FIG.19(c) is an explanatory diagram plotting the relationship between shortside length b and shear buckling stress τcr.

FIG. 20 is a perspective view showing a body structure for rolling stockaccording to a second embodiment of the present invention.

FIG. 21 is an explanatory view illustrating the relation between anoutside sheathing and outside sheathing reinforcement members.

FIGS. 22(a) and 22(b) are each a view showing an embodiment in whichoutside sheathing reinforcement member are provided at their endportions with a small-type reinforcement member; and FIG. 22(c) is aperspective view showing a variation of the outside sheathingreinforcement member.

FIG. 23(a) is a perspective view showing a corner part of an openingportion;

FIG. 23(b) is a perspective view showing another example of a cornerpart of the opening portion; and FIG. 23(c) is a perspective viewshowing yet another example of a corner part of the opening portion.

FIG. 24(a) is a perspective view showing another example of a cornerpart of the opening portion; FIG. 24(b) is a perspective view showinganother example of a corner part of the opening portion; FIG. 24(c) is aperspective view showing another example of a corner part of the openingportion; FIG. 24(d) is a perspective view showing another example of acorner part of the opening portion.

FIG. 25(a) is a perspective view showing a joint portion between a sideoutside sheathing and a roof outside sheathing; and FIG. 25(b) is aperspective view showing another example of a joint portion between theside outside sheathing and the roof outside sheathing.

FIG. 26(a) and FIG. 26(b) are each a perspective view showing a jointportion between a side outside sheathing and an end outside sheathing.

FIG. 27 is a perspective view showing a joint portion between a sideoutside sheathing (wainscot sheathing) and a side beam.

FIG. 28(a) is an explanatory view illustrating a joint portion of a sideoutside sheathing; and FIGS. 28(b) and 28(c) are each an explanatoryview illustrating a joint portion of a side outside sheathing.

FIG. 29 is a perspective view showing a joint portion between a sideoutside sheathing and an edging member.

FIG. 30 is a perspective view showing a joint portion between a sideoutside sheathing and an edging member.

FIG. 31(a) is an explanatory view illustrating a method of joining anoutside sheathing reinforcement member; and FIG. 31(b) is an explanatoryview illustrating another method of joining an outside sheathingreinforcement member.

FIG. 32 is a perspective view showing another example of a corner partof the opening portion.

FIG. 33(a) is a perspective view showing another example of a cornerpart of the opening portion; and FIG. 33(b) is an explanatory viewillustrating a welding method.

FIGS. 34(a) and 34(b) are each an explanatory view showing a mountingstructure for interior trim or equipment.

FIG. 35(a) is a detail view of a portion A shown in FIG. 24; and FIG.35(b) is a detail view of a portion B shown in FIG. 24.

FIGS. 36(a) and 36(b) are explanatory views illustrating a method ofassembling a side construction.

FIGS. 37(a) and 37(b) are explanatory views illustrating a method ofassembling a side construction.

FIGS. 38(a) and 38(b) are explanatory views illustrating a method ofassembling a side construction.

FIGS. 39(a) and 39(b) are explanatory views illustrating a method ofassembling a side construction.

FIG. 40 is an explanatory view illustrating one exemplary procedure forassembling a side construction for rolling stock according to thepresent invention.

FIG. 41(a) is an explanatory view illustrating the principle underlyinga laser-welded joint according to the present invention; and FIGS. 41(b)to 41(d) are each an explanatory view illustrating an outside sheathingpanel according to the present invention.

FIG. 42 is a view showing a side construction for rolling stockaccording to the second embodiment of the present invention as viewedfrom the outside of the railway car.

FIG. 43 is a view showing the side construction for rolling stock asviewed from the inside of the railway car.

FIG. 44 is a perspective view showing the side construction as viewedfrom the inside of the railway car.

FIGS. 45(a) and 45(b) are each an explanatory view illustrating aconventional stainless steel body structure.

FIGS. 46(a) and 46(b) are each an explanatory view illustrating aconventional body structure for rolling stock in a deformed condition;and FIG. 46(c) is an explanatory view illustrating the relation betweenan outside sheathing and outside sheathing reinforcement membersaccording to a conventional technique.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings.

First Embodiment

FIG. 2 illustrates a basic principle based on which a body structure forrolling stock is constructed using a laser-welded joint according to thepresent invention.

As shown in FIGS. 2(a) and 2(b), partial penetration laser welding isused to join first reinforcement members 12 each substantially shapedlike a hat in section to an inside surface of an outside sheathing 11 (asurface ground material having an outside surface 11 a previouslypolished by a belt grinder substantially in parallel with the directionof a weld line to be formed by laser welding) so that the bottom of aresulting molten pool will not reach the outside surface 11 a of theoutside sheathing 11. At that time, the outside surface 11 a of theoutside sheathing 11 has been previously polished substantially inparallel with the direction of a weld line to be formed by laserwelding. That is, the direction of the weld line to be formed by laserwelding and the direction in which the outside surface 11 a has beenpolished (the direction in which the ground surface finishing is made)substantially coincide with each other.

Each of the first reinforcement members 12 is shaped like a hat insection and has a body part 12 a substantially U-shaped in section anddefining therein a space gradually widening as it extends toward theopening side, and flange parts 12 b and 12 c to be joined to the outsidesheathing 11 by laser welding, the flange parts 12 b and 12 c extendingcontinuously from opening defining edges of the body part 12 a inopposite directions. The body part 12 a has a base plate portion 12 aaextending substantially in parallel with the outside sheathing 11 in ajoined condition, and side plate portions 12 ab and 12 ac extending fromthe opposite side edges of the base plate portion 12 aa toward theopening defining edges in such a manner that the spacing therebetweenwidens as it extends toward the opening defining edges.

Since the body part 12 a is substantially U-shaped in section in such amanner that the spacing between the side plate portions 12 ab and 12 acwidens gradually as it extends toward the flange parts 12 b and 12 c,each first reinforcement member 12 is hard to deform (shear deformation)by a shear load working in a direction perpendicular to the firstreinforcement member 12 and parallel with the outside sheathing plane.Accordingly, when a high antiplane flexural stiffness is required alsoin the direction perpendicular to the first reinforcement member 12 tosuppress sectional deformation as in cases where the first reinforcementmember 12 is used in an outside sheathing panel of a body structure forrolling stock, it is possible to obtain such a high antiplane flexuralstiffness by joining a second reinforcement member 13 having ahat-shaped section to the upper side of the first reinforcement member12 (body part 12 a) perpendicularly thereto by laser welding. This isbecause the first reinforcement member 12 can suppress shear deformationthat occurs between the second reinforcement member 13 and the outsidesheathing and hence can prevent the antiplane shear stiffness of theoutside sheathing panel from lowering, which results in the outsidesheathing panel having an antiplane flexural stiffness kept high againstany load. By so doing, the antiplane flexural stiffness of the outsidesheathing panel can be kept satisfactory without the need to join thesecond reinforcement member 13 directly to the outside sheathing 11 and,hence, the outside sheathing 11 can be kept aesthetically satisfactory.

The second reinforcement member 13 also has a body part 13 asubstantially U-shaped in section and flange parts 13 b and 13 c to bejoined to the outside sheathing 11 by laser welding, the flange parts 12b and 12 c extending continuously from opening defining edges of thebody part 13 a in opposite directions, as shown in FIG. 2(c).

When use is made of a member, like the second reinforcement member 13,to be formed with a weld line extending in a direction that is notcoincident with the surface polishing direction (ground surfacefinishing direction) on the outside surface of the outside sheathing 11,laser welding should be conducted to join the second reinforcementmember 13 to the first reinforcement member 12 (body part 12 aa)directly joined to the outside sheathing 11 so as to prevent a resultingweld mark from being exposed on the outward appearance (outsidesurface). Since the weld mark is not exposed at a place that can beviewed from the outside, the laser welding conducted in this case can befull penetration laser welding which is easy to control during weldingunlike the laser welding conducted to join the outside sheathing 11 andthe first reinforcement member 12 (flange parts 12 b and 12 c) together.By thus employing partial penetration laser welding and full penetrationlaser welding separately in constructing the entire structure by laserwelding, the entire structure can be imparted with required jointstrength easily without impairment of its outward appearance.

With reference to FIGS. 3 to 14, description will be made of a case inwhich such a laser-welded joint or outside sheathing panel is used in abody structure for rolling stock

As shown in FIG. 3, an outside sheathing 21 (having a thickness of 1.5mm for example) comprises three parts including a wainscot sheathingpart 21A, a pier panel/window periphery part 21B, and a frieze boardpart 21C, which have their respective end edges lapped over the otherand joined together by partial penetration laser welding.

These parts 21A to 21C of the outside sheathing 21 each has an outsidesurface previously polished in a direction coinciding with thelongitudinal direction of a railway car (rail direction) and formed witha weld line extending parallel with the longitudinal direction of therailway car by laser beam. Reference numerals 22 and 23 denote a windowopening portion and a door opening portion, respectively.

As shown in FIGS. 4 to 9 and 12 to 14, the parts 21A to 21C of theoutside sheathing 21 are interiorly joined with inside plate panels(each having a thickness of 1 mm for example) 25 to 29 extendingparallel in the longitudinal direction of the railway car by partialpenetration laser welding, each of the inside plate panels 25 to 29being shaped like a hat in section. That is, the inside plate panels 25to 29 extend in the longitudinal direction of the railway car, whichcoincides with the direction of the weld line formed by laser welding.

As shown in FIG. 4, the wainscot sheathing part 21A of the outsidesheathing 21 is interiorly provided with four inside plate panels 25each shaped like a hat in section and extending in the longitudinaldirection of the railway car. The inside plate panels 25 are verticallyarranged in parallel at predetermined pitches. The pier panel/windowperiphery part 21B (of the outside sheathing) is interiorly providedwith two inside plate panels 26 each having a ridge portion, four insideplate panels 27 each shaped like a hat in section, and two inside platepanels 28 each having a ridge portion, which are arranged in theascending order on each of the front and rear sides of the windowopening portion 22. All the inside plate panels 26, 27 and 28 arevertically arranged in parallel at predetermined pitches so as to extendin the longitudinal direction of the railway car. The frieze board part21C of the outside sheathing 21 is interiorly provided with two insideplate panels 29 which are vertically arranged in parallel with apredetermined spacing therebetween so as to extend in the longitudinaldirection of the railway car.

On the front and rear sides of the window opening portion 22, door endposts 31 each shaped like a hat in section and extending in a directionperpendicular to the inside plate panels 25 to 29 are laid over theinside plate panels 25 to 29 on the inner side in the railway car. Eachof the door end posts 31 has flange parts which are joined to the bodyparts of the inside plate panels 25 to 29 by full penetration laserwelding by which the bottom of a molten pool penetrates through each ofthe inside plate panels 25 to 29. In this way, a portion around the sideopening is stiffened. Each of the door end posts 31 corresponds to theaforementioned second reinforcement member and has a body part 31 ahaving a U-shaped section and flange parts 31 b and 31 c joined to theinside plate panels 25 to 29 by laser welding and extending continuouslyfrom the body part 31 a in opposite directions, as shown in FIGS. 5 to11.

Also, a window post 32 shaped like a hat in section is provided underthe window opening portion 22 as shown in FIG. 4. The window post 31 hasa structure similar to the aforementioned door end post 31, having abody part 32 a having a U-shaped section and flange parts 32 b and 32 cextending continuously from the body part 32 a in opposite directions,the flange parts 32 b and 32 c being joined to the body parts of theinside plate panels 25 and the like by full penetration laser welding bywhich the bottom of a molten pool penetrates through each of the insideplate panels 25 or the like.

Each of the above-described internal panels 25 to 29 corresponds to theaforementioned first reinforcement member 12 and is shaped like a hat insection to define therein a space gradually widening as it extendstoward the opening as shown in FIG. 6. For example, the inside platepanel 25 has a body part 25 a U-shaped in section to define therein aspace gradually widening as it extends toward the opening, and flangeparts 25 b and 25 c joined to the outside sheathing 21A by laser weldingand extending continuously from the body part 25 a in oppositedirections, as shown in FIG. 8.

As shown in FIG. 4, a belt rail 33 and a window head 34 are joined torespective portions lying under and above the window opening portion 22by laser welding. Like the inside plate panels 25 to 29, the belt rail33 and the window head 34 are each shaped like a hat in section todefine therein a space gradually widening as it extends toward theopening as shown in FIGS. 8 and 9. Reference numerals 24A and 24B denotea window frame and a doorframe, respectively.

As shown in FIG. 4 again, the window opening portion 22 has four cornersprovided with respective joints 37, and each of the lower joints 37 isopposed to another joint 36 across the adjacent door end post 31.

The inside plate panels 25 to 29 are joined to the outside sheathing 21in such a manner that: the inside plate panels 25 to 29 are overlaid onthe inside surface of the outside sheathing 21; and the resultingstructure is continuously irradiated with moving laser beam from theinside plate panel side so as to be heated until a region extending toan inner portion of the outside sheathing 21 becomes molten with thelaser power or moving speed of the laser beam controlled so as not toallow a bottom portion of a resulting molten pool to reach an outsidesurface of the outside sheathing 21 (partial penetration laser welding).Such unification of the type of joint between the outside sheathing 21and the inside plate panels 25 to 29 (partial penetration laser-weldedjoint) makes it possible to stabilize the welding operation. Further,since the outside sheathing 21 is supported by highly stiff frameworkmembers (inside plate panels 25 to 29, door end posts 31, window post 32and the like) against antiplane deformation, the accuracy of sectionaldimensions can be improved.

Any one of the outside sheathings 21 (21A to 21C) is a plate memberhaving an outside surface polished (surface-finished) in thelongitudinal direction of the railway car which is substantiallyparallel with the direction of a weld line to be formed by theaforementioned laser beam. The polishing is desirably BG (belt grinder)finishing using a belt grinder. A trial model of a side construction wasconstructed using, for example, SUS304#80BG material, which isfrequently used as an outside sheathing for rolling stock for domesticold railroad lines. In the case where inside plate panels (each having athickness of 1 mm) were laser-welded to an outside sheathing from theupper plate side at a laser power of 2.5 kW, the relationship betweenthe width of the joint portion between the upper and lower plates andthe tensile shear strength was proved to be as shown in FIG. 15. As canbe understood from FIG. 15, if the welding speed is controlled to adjustthe joint width w to about 0.7 mm, a required joint strength (a tensileshear strength of about 5 kN/cm) can be ensured. Because scorch resultsfrom welding if the upper plate is not thinner than the lower plate, itcan be estimated from various experiments that the plate thickness ofthe lower plate need be 1.5 times or more as large as the platethickness of the upper plate (for example, a combination of an upperplate thickness of 1 mm and a lower plate thickness of 1.5 mm or acombination of an upper plate thickness of 1 mm and a lower platethickness of 2.0 mm).

A side construction for rolling stock may be constructed in a differentmanner than described above in which the inside plate panels each shapedlike a hat in section are replaced with an inside plate panel formedwith repeated groove and ridge (made by joining inside plate panels eachhaving a hat-shaped section one to another contiguously), as shown inFIG. 16.

In this case, outside sheathings 41A, 41B and 41C, which correspond tothe frieze board part, pier panel/window periphery part and wainscotsheathing part, respectively, are each interiorly joined with arespective one of inside plate panels 42A, 42B and 42C each having asize adjusted to a respective one of the outside sheathings by partialpenetration laser welding in the longitudinal direction of the railwaycar to form outside sheathing panels 43A, 43B and 43C for the friezeboard part, pier panel/window periphery part and wainscot sheathingpart, respectively. In this case, the groove portions (portionscontacting the inside surfaces of the outside sheathings 41A to 41C) ofthe inside plate panels 42A to 42C extend in the longitudinal directionof the railway car and these portions are joined to the outsidesheathings 41A to 41 c by partial penetration laser welding.

Subsequently, the outside sheathings 43A, 43B and 43C for respective ofthe frieze board part, pier panel/window periphery part and wainscotsheathing part are joined to each other to have predetermined positionalrelation by partial penetration laser welding in the longitudinaldirection of the railway car, thus forming a construction 49′.

Thereafter, window heads 44A and 44B and a belt rail 45 are joined toportions lying above and under the window opening portion by partialpenetration laser welding in the longitudinal direction of the railwaycar, thus forming a construction 49″. Further, the construction 49″ isjoined with door end posts 46, window post 47 and doorframes 48 toensure a required stiffness, thus forming a side construction 49.

Second Embodiment

FIG. 20 is a perspective view showing a body structure for rolling stockaccording to a second embodiment of the present invention.

As shown in FIG. 20, the body structure 301 for rolling stock includesright and left side constructions 302, a roof construction 303, frontand rear end constructions 304, and an underframe 305. Each of the sideconstructions 302 has an outside sheathing 306 and outside sheathingreinforcement members 307A and 307B (third reinforcement members) joinedto the inside surface of the outside sheathing 306 by laser welding.(The same holds true for the roof construction 303 and the endconstructions 304.) The outside sheathing reinforcement member 307A (or307B) is shaped like a hat in section having a U-shaped part 307Aahaving a U-shaped section, and fitting parts 307Ab and 307Ac extendingcontinuously from the opposite end edges of the U-shaped part 307Aa inopposite directions. These fitting parts 307Ab and 307Ac arelaser-welded to the outside sheathing 306.

A laser device (not shown) used for laser welding is attached to a wristof a multijoint robot. This multijoint robot has plural axes (six axesfor example) and is capable of moving the laser device along apredetermined weld line U on a to-be-welded thin plate (for example,outside sheathing 306) in a fixed condition. Such a laser device forlaser lap welding is integrated with a laser focusing device andpressing means. This laser device presses the pressing means having, forexample, a roller (hold-down roller 331) against a portion of a workadjacent a weld position while moving the pressing means along the weldline U and applies a laser beam 312 through the laser focusing device tothe weld position. Thus, the laser device is configured to be capable ofaccurately keeping a focal length of the laser beam while ensuringintimate contact between lapped portions.

The outside sheathing 306 and an edging member 308 (such as a windowframe or a door mask) to be provided on the peripheral edge of anopening portion (window opening portion S1 or a doorway opening portionS2) are joined together by continuous laser welding. The joint formed inthis case may be a lap joint or a butt joint. Though not illustrated indetail, the outside sheathings are joined to each other by continuouslaser welding.

Meanwhile, from a stress analysis on a conventional railway car in whichan in-plane stress component exerted on the outside sheathing 306 isresolved into uniaxial stresses σx and σy in respective of the raildirection and a direction perpendicular thereto and shear stress τ, itcan be estimated that stresses σx, σy and τ working at a corner of thewindow opening portion S1 are about 130, 50 and 35 Mpa, respectively.Substitution of the values of stresses σx, σy and τ thus estimated intothe aforenoted formula (II) yields b=min(104, 87, 255)=87 mm as atheoretical value of pitch b of the reinforcement members 307 A to bearranged in the vicinity of the corner of the window opening portion S1.Actually, for the structure to have a slightly higher strength forsafety, the pitch b of the outside sheathing reinforcement members 307Aarranged in the vicinity of the corner of the window opening portion Sis set to 80 mm. From this fact, it can be said that the pitch b ofoutside sheathing reinforcement members 307A defined by the spacingbetween adjacent weld lines U formed by laser welding is determinedbased on the buckling strength of a portion at which the load imposed onthe outside sheathing reinforcement members 307A becomes maximized inthe rail direction in which the outside sheathing reinforcement members307A extend.

By setting the spacing L1 defined by laser welding (weld lines U) to 80mm (hat width L2=50 mm) as shown in FIG. 21, the aforementioned pitch bof outside sheathing reinforcement members 307A can be set to 80 mm. Thehat width L2 of each of outside sheathing reinforcement members providedon other part of the outside sheathing 306 and the laser welding spacingL1 associated therewith are established in the same manner as describedabove. For example, the hat width L2 of each of outside sheathingreinforcement members 307B and the laser welding spacing L1 are set to70 mm and 100 mm, respectively. Laser welding spacing L3 between outsidesheathing reinforcement members 307A and 307B positioned adjacent toeach other is appropriately established in accordance with the shapesand sizes of the outside sheathing reinforcement members 307A and 307B,the laser welding spacing L1 and the like. In this embodiment, thespacing L3 is set to 20 to 50 mm. The height H of the outside sheathingreinforcement members 307A and 307B is appropriately established basedon the general buckling strength of the outside sheathing 306 of theside construction 302. The height H established here is 25 mm.

If the sectional shape of an outside sheathing reinforcement member isdetermined part by part of outside sheathing 306, there will be anincrease in the number of types of outside sheathing reinforcementmembers to be used. For this reason, the sectional shapes of the outsidesheathing reinforcement members used in this embodiment are limited totwo types.

In this way, the side construction is formed in which the hat width L2(the width of the U-shaped part) of each of the outside sheathingreinforcement members 307A provided in the vicinity of each corner ofthe window opening portion S1 and in the vicinity of each of the uppercorners of the doorway opening S2 is set to 50 mm while the hat width ofeach of the outside sheathing reinforcement members 307B provided onother part set to 70 mm. Thus, the hat width of each of the outsidesheathing reinforcement members 307A provided in the vicinity of eachcorner of the window opening portion S1 and in the vicinity of each ofthe upper corners of the doorway opening S2 where the stress exerted ishigh and the load imposed is large, is set larger than the hat width ofeach of the outside sheathing reinforcement members 307B provided onother part. The outside sheathing is stiffened by stringers providedseparately against antiplane flexure in the direction perpendicular tothe rail direction.

The outside sheathing 306 and each of the outside sheathingreinforcement members 307A and 307B define a closed space therebetween.Since the portion of the outside sheathing 306 and/or the portion ofeach of the outside sheathing reinforcement members 307A and 307B, whichare located near the entrance of the closed space, are not reinforced,the outside sheathing 306 of the side construction to be subjected torelatively bad load conditions and the like might buckle in thedirection perpendicular to the direction in which the outside sheathingreinforcement members 307A and 307B extend. For the purpose ofreinforcing the portion of the outside sheathing 306 and/or the portionof each of the outside sheathing reinforcement members 307A and 307Bwhich are located near the entrance of the closed space, a small-sizedreinforcement plate may be joined to a portion located coincidentallywith the closed space prior to the formation of the closed space byjoining of the outside sheathing reinforcement member 307A or 307B tothe outside sheathing 306. For the workability to be enhanced,structures as shown in FIGS. 22(a) to 22(b) may be employed.

If a small-sized reinforcement member 311 (a fourth reinforcementmember) having a U-shaped section is previously provided inside theoutside sheathing reinforcement member 307 having a hat-shaped section,local reinforcement can be achieved. That is, the two reinforcementmembers 307 and 311 are simply integrated together previously bypositioning the reinforcement member 311 in an end portion of theoutside sheathing reinforcement member 307 and joining opposite sideportions 311 a of the reinforcement member 311 to respective of theopposite side portions of the U-shaped part 307 a of the outsidesheathing reinforcement member 307 by laser welding along the weld lineU.

With such a structure, the reinforcement member 311 can also be mountedon the outside sheathing 306 by merely joining the outside sheathingreinforcement member 307 to the outside sheathing 306 by laser welding.Also, the opening portion of the outside sheathing reinforcement membercan be reinforced and, hence, necessary and minimum reinforcement can beprovided on a portion requiring reinforcement without the need toincrease the number of processes.

In addition to the above-described reinforcement, the bottom portion ofthe reinforcement member 311 may be joined to the outside sheathing 306by means of a laser beam 312 of laser welding (or a spot-welding gun)through an opening 307 b (or a notch) formed in the outside sheathingreinforcement member 307 (the head portion of the U-shaped part 307 a),as shown in FIG. 22(b). By so doing, the buckling strength of theoutside sheathing 306 can be further enhanced.

Usually, the aforementioned outside sheathing reinforcement member 307Ashaped like a hat in section is formed by bending a flat plate materialinto a V-shape by means of a press. Since the outside sheathingreinforcement member 307A has a two-dimensional section, an end portionof the outside sheathing reinforcement member 307A reaching the openingportion S1 or S2 of a side window or a side doorway forms an free edgeexposing the closed section and, hence, the end portion might beweakened in stiffness. (The “free edge” means an exposed end portion ofa member extending in a specific direction with its sectional shape heldconstant, the exposed end portion being free of any reinforcement orthree-dimensional working.)

In view of this disadvantage, the aforementioned outside sheathingreinforcement member may be replaced with a reinforcement member 313(with closed ends) having a root portion 313 a and a hollow bulgedportion 313 b formed by drawing a central portion of the root portion313 a, as shown in FIG. 22(c). The root portion 313 a is joined to theoutside sheathing 306 by laser welding along a weld line U extendingover the entire periphery of the root portion 313 a as indicated by along dashed short dashed line. This reinforcement member 313 can haveenhanced buckling strength and stiffness at its ends. If continuouswelding such as laser welding, in particular, is employed, the peripheryof each end portion of the reinforcement member 313 can be closedcompletely. That is, since no free edge is formed at the end portions ofthe reinforcement member 313, it is possible to further enhance thestiffness of the end portions of the reinforcement member 313.

In cases where the V-bent pressed material is used as the outsidesheathing reinforcement member as described above, it is preferable thatan inside plate frame 314 in the form of flat plate is placed over theU-shaped parts 307 a of outside sheathing reinforcement members 307arranged on the peripheral edge portion of the opening portion S1 of,for example, a side window and then joined thereto by laser welding tointerconnect the (free-edge) end portions of the outside sheathingreinforcement members 307. By so doing, the end portions of the outsidesheathing reinforcement members 307 can be reinforced easily.

The inside plate frame 314 in the form of flat plate may be replacedwith an inside plate frame 315 or 316 having a certain height as shownin FIGS. 23(b) and 23(c). In this case, the inside plate frame 315 or316 has a portion 315 a or 316 a to be laser-welded to the outsidesheathing reinforcement members 307, and a portion 315 b or 316 b havingan L-shaped section which extends continuously from the portion 315 a or315 b on the side opposite away from the outside sheathing 306. Such aninside plate frame 315 or 316 has a higher stiffness than the insideplate frame 314 in the form of flat plate. As a result, it is possibleto further enhance the stiffness of the end portions of the outsidesheathing reinforcement members 307.

The above-described structures (see FIGS. 23(a) to 23(c)) are applicablenot only to the peripheral edge portion of the opening portion S1 of aside window of the outside sheathing 306 but also to a corner portion ofother opening portion.

If the inside plate frame 314 placed over the U-shaped parts 307 a ofthe outside sheathing reinforcement members 307 located adjacent such anopening portion forms a free edge as shown in FIG. 23(b) or 23(c), it isdifficult for shearing force to be decentralized over the inside plateframe 314 and, hence, the aforementioned opening portion sometimes haveinsufficient strength at its corners in particular.

For this reason, it is desirable that an inside plate frame 314A have abody part placed over the U-shaped parts 307 a of the outside sheathingreinforcement members 307, a leg part 314 a extending continuously fromthe body part of the inside plate frame 314A down to the outsidesheathing 306 so as to close the open ends of the outside sheathingreinforcement members 307, and a flange part 314 b extendingcontinuously from the leg part 314 a in the direction perpendicular tothe leg part 314 a for direct joint to the outside sheathing 306 bylaser welding, as shown in FIG. 24(a). Such a structure makes itpossible to enhance the stiffness and strength of the end portions ofthe outside sheathing reinforcement members 307 because the inside plateframe 314 does not form any free edge as shown in FIG. 23(b) or 23(c).This structure is applicable to the corners of the opening portion S1.If the periphery of the opening portion S1 is reinforced with the insideplate frame 314A integrated with the leg part 314 a and the flange part314 b, the stiffness of the opening portion and the strength of thecorners can be further enhanced.

If the inside plate frame 314A for reinforcement of the peripheral edgeof the opening portion S1 is mounted after the outside sheathingreinforcement members 307 each having a hat-shaped section have beenjoined to the outside sheathing 306, it is possible to avoidinterference between the inside plate frame 314A and the hold-downroller to be described later.

A portion 314 c of the leg part 14 a of the inside plate frame 314B,which corresponds to a corner of the opening portion S1, may be curvedby drawing or the like as shown in FIG. 24(b). Such a feature allows theinside plate frame 314B to impart the corner of the opening portion S1with a sufficient strength assuredly.

When the outside sheathing reinforcement members 307 are required tohave enhanced antiplane stiffness, an inside plate frame 314C havingunequal leg sections may be used instead of the inside plate frame 314Aor 314B. As shown in FIG. 24(c), the inside plate frame 314C comprises afirst leg part 314 d, a first flange part 314 e extending continuouslyfrom and perpendicularly to the lower end edge of the first leg part 314d, a second leg part 314 f formed to have a longer leg length than thefirst leg part 314 d and extending substantially parallel with the firstleg part 314 d with a predetermined spacing therefrom, a second flangepart 314 g extending continuously from and perpendicularly to the lowerend edge of the second leg part 314 f, and a horizontal part 314 hbridging the upper end edge of the first leg part 314 d and the secondleg part 314 f. The first and second flange parts 314 e and 314 g extendaway from each other. The inside plate frame 314C is configured suchthat in a condition where the first flange part 314 e and the secondflange part 314 g are joined to the outside sheathing reinforcementmembers 307 (U-shaped parts 307 a) and the outside sheathing 306,respectively, the horizontal part 314 h is positioned to extendsubstantially parallel with the upper surfaces of the U-shaped parts ofthe reinforcement members 307 with a predetermined spacing therefrom.Such a feature makes it possible to enhance the antiplane stiffness ofthe outside sheathing reinforcement members 307.

In this case also, a portion 314 k of the inside plate frame 314D, whichcorresponds to a corner of the opening portion, may be curved as shownin FIG. 24(b). Such a feature can enhance the strength of the corner ofthe opening portion S1.

The structure thus constructed provides merits including: (i) use oflaser welding for joining various elements makes the outside sheathingfree from decrease in buckling strength due to buckling between adjacentwelded spots or initial strain and, hence, the structure can enjoy abuckling strength as designed; (i) accordingly, if the pitch of theoutside sheathing reinforcement members is determined theoretically,there is no need to provide the outside sheathing with additional smallreinforcement plates for further reinforcement, which can contributes toreduction in parts count and in man-hour; (iii) only two types ofoutside sheathing reinforcement members are used, the number of which ismarkedly smaller than required by the conventional art, thus leading toa reduction in the parts manufacturing cost; and (iv) the decreasednumber of types of parts makes it easy to improve the dimensionalaccuracy of the parts.

Since lap portions are metallurgically joined together continuously bycontinuous laser welding, a sealant as a separate part can be eliminatedand the joint does not allow penetration of water therethrough due toaged deterioration. This is because laser welding can hardly causethermal strain to occur by virtue of its high energy density, unlike arcwelding.

By utilizing the feature that watertightness can be ensured by merelyconducting continuous laser welding without provision of any sealant, itis possible to form a water channel such as rainwater guttering by theuse of only the structural members of the body structure without theneed to provide any additional water tube or rainwater guttering.

Specifically, a side outside sheathing and a roof outside sheathing arejoined together by continuous laser welding. At that time, usually, atleast one of the side outside sheathing and the roof outside sheathinghas an edge portion that is folded to form a folded part as stiffnessenhancing means for receiving counterforce produced during laserwelding. As shown in FIG. 25(a), for example, an end portion of a sideoutside sheathing 306A (frieze board) of a side construction is foldedinwardly to form a folded part 306Aa as a counterforce receiving parthaving a downwardly open U-shaped section for receiving counterforceproduced during laser welding. An end portion of a roof outsidesheathing 306B (cantrail) of a roof construction is folded outwardly toform a folded part 306Ba as a rainwater guttering part having anupwardly open U-shaped section. The upper surface of the folded part306Aa of the side construction and the lower surface of the folded part306Ba of the roof construction are lapped over the other, and the lapportions of the two are joined together by laser welding along a weldline U. By so doing, it is possible to ensure required stiffiness aswell as to form the rainwater guttering part (folded part 306Ba).

As shown in FIG. 25(b), it is possible that: the extent of the sideconstruction is extended up to the roof outside sheathing (cantrail)lying above the side outside sheathing 306A (frieze board); and an endportion 306Ab of the side outside sheathing 306A is folded upwardlywhile an end portion 306Bb of the roof outside sheathing 306B to bejoined with the end portion 306Ab folded outwardly to form acounterforce receiving part. Since this counterforce receiving part hasan upwardly open U-shaped section, the counterforce receiving part, asit is, can be used as rainwater guttering.

As shown in FIGS. 26(a) and 26(b), the side construction and the endconstruction are joined together by joining a side outside sheathing 323to an end corner post 322 by continuous laser welding using the endcorner post 322 as a counterforce receiver, the end corner post 322having a substantially L-shaped horizontal section and having beenpreviously joined with an end outside sheathing 321 of the endconstruction. The end outside sheathing 321 and the side outsidesheathing 323 are positioned to extend perpendicular to each other.

In this case, if a separate member 324 is provided such as to define aclosed space cooperatively with the end corner post 322 as shown in FIG.26(b), a rainwater guttering can be formed for guiding rainwater fromthe roof to the ground. Thus, there is no need to provide an additionalwater tube for guiding rainwater from the roof to the ground. The endcorner post 322 has a first face plate part 322 a to be joined to theend outside sheathing 321, and a second face plate part 322 b to bejoined to the side outside sheathing 323. The member 324 has flangeparts 324 a and 324 b to be joined to the face plate parts 322 a and 322b, respectively, of the end corner post 322 and a curved body part 324 cintervening between the flange parts 324 a and 324 b. The body part 324c and the end corner post 322 define therebetween the closed space foruse as a water channel (rainwater guttering).

Such a closed space may be defined between the end corner post 322A andthe end outside sheathing 321 as shown in FIG. 26(b). In this case, aclosed space defining part 322 c having a hat-shaped section iscontinuous with the end corner post 322A on the end outside sheathing321 side. Flange parts 322 d and 322 e on opposite sides of the closedspace defining part 322 c are continuously laser-welded to the endoutside sheathing 321, thereby ensuring watertightness.

As shown in FIG. 27, the side outside sheathing 323 and the underframe(side beam 325) are joined together by joining a lower portion (wainscotsheathing) of the side outside sheathing 323 to the side beam 325 havinga U-shaped section by continuous laser welding using the side beam 325as a counterforce receiver.

With respect to the side outside sheathing, a lap joint is formed byjoining lapped end portions of respective outside sheathings 323A and323B together or lapped portions of respective of the outside sheathing323B and a frame member 326 (edging member of an opening portion)together by continuous laser welding. Because continuous laser weldingis performed on construction units of respective of the outsidesheathings 323A and 323B or construction units of respective of theoutside sheathing 323B and the frame member 326 in a condition set on ajig prior to the assemblage of the body structure, there is no need toprovide any counterforce receiver. Beside such a lap joint, a butt jointJ1 and lap fillet weld joint as shown in FIGS. 28(b) and 28(c) arepossible.

In laser-welding the outside sheathing 323B and the frame member 326(edging member of the opening portion) to each other, if the thicknessof the plate material forming the frame member 326 is comparable to orsmaller than that of the outside sheathing 323B, weld scorch and edgebend occur on the surface of the outside sheathing 323 b and, as thecase may be, the weld bead is pierced through. In such a case, a weldmark appears around the opening portion, thus impairing the outwardappearance in aesthetic terms.

To avoid such an inconvenience, the edging member 326 located on theoutside surface, such as a window frame or a door mask, may comprise athick plate that is thicker than the outside sheathing 306 as shown inFIG. 29. If the plate thickness of the edging member 326 located on theoutside surface is not less than 3 mm, the weld line U can be renderedsubstantially invisible irrespective of the type and condition ofsurface finishing.

If the plate material forming the edging member 326 forming the lapjoint is rolled to form a roll part 327 a defining a space therewithinas shown in FIG. 30, the outside surface of the structure will not beaffected even by piercing welding. Such a roll part 327 a can be formedby drawing or roll diffusion bonding or by V-bending a plate material bypress or pressing a steel tube. Within the roll part 327 a, inner platesurfaces may be brought into contact with each other without definingthe space. The space need not necessarily be defined within the rollpart 327 a, but it is sufficient for the portion of the edging member327 joined to the outside sheathing 306 to have a folded plate structureformed by folding a flat plate.

By so doing, it is possible to retain the watertightness with anunnecessary weight increase suppressed, as well as to further enhancethe design characteristics. If three plates have to be lapped one overanother in the manner of frame+outside sheathing+framework member, theroll part thus structured, which is free from being pierced through,allows lap welding of the three plates at a time.

It is possible to eliminate, for example, a window frame if the outsidesheathing of a pier panel part comprises a thick plate (not less than 3mm) as described above. In this case, the parts count can be lowered.

In laser lap welding, a hold-down roller 331 (having a diameter of about100 mm), which is movable together with a laser beam, is used to ensureintimate contact between the lap portions as well as to keep the focallength of the laser beam accurate. This hold-down roller 331 isconfigured to move together with the laser beam and press a portion ofreinforcement member 307 to be joined to the outside sheathing 306against the outside sheathing 306 during laser-welding of thereinforcement member 307 to the outside sheathing 306.

If the edging member 332 (a frame such as a window frame or a door mask)is attached to end portions of the outside sheathing reinforcementmembers 307 before laser welding, the edging member 332 interferes withthe hold-down roller 331 and hence does not allow continuous laserwelding up to such an end portion to be achieved. For this reason, ithas been a conventional practice to perform welding with the hold-downroller 331 removed, which requires very much man-hour. On the contrary,if the edging member 332 is attached after laser welding, the outsidesheathing reinforcement member 307 might interfere with the hold-downroller 331, thus making continuous laser welding impossible.

To avoid such an inconvenience, the edging member 332 may be formed witha notch 332 b which enables the hold-down roller 331 to advance to alocation adjacent the end portion of the outside sheathing reinforcementmember 307, the notch 332 b being formed by cutting off only the portion(of flange 332 a or the like) of the edging member 322 in which thehold-down roller 331 is expected to pass. This feature allows theoutside sheathing 306 and the outside sheathing reinforcement member 307to be laser-welded to each other quickly and accurately in one stepwithout the need to remove the hold-down roller 331.

Alternatively, it is possible to place an elongate hold-down jig 333having a larger height than the edging member 332 on the outsidesheathing reinforcement member 307 and hold down the outside sheathingreinforcement member 307 from above the jig 333 by means of thehold-down roller 331 acting on the outside sheathing reinforcementmember 307 through the jig 333, as shown in FIG. 31(b).

This feature has a merit that the outside sheathing reinforcement member307 can be welded to the outside sheathing 6 up to the end portion ofthe reinforcement member 307 without modifying any structural member.

In order to obtain a stainless steel body structure having ahigh-precision outside surface of aesthetic value, the outside surfaceof the outside sheathing is beautifully finished by brushing or shotblasting after assemblage of the outside sheathing panel by continuouslaser welding. Alternatively, it is possible that the outside sheathingis beautifully finished by brushing or shot blasting after assemblage ofthe body structure by laser welding. Otherwise, weld lines may becovered with color bands. The laser-welded joint may be formed by anyone of piercing lap welding, partial penetration lap welding and buttwelding.

If the plate thickness of the outside sheathing 306 is not less than 3mm, partial penetration laser welding can be performed with no weld lineappearing on the outside surface regardless of the surface finishing ofthe outside sheathing and the weld line direction.

By merely conducting such partial penetration laser welding, the highaesthetic value of the outside sheathing can be ensured without the needto conduct any post-finishing such as brushing.

In another embodiment, weld lines on the outside sheathing are made toextend in a fixed direction, particularly, weld lines on the sideoutside sheathing made to extend in the longitudinal direction of therailway car. By so doing, somewhat visible weld marks resulting on thesurface of the outside sheathing can be rendered inconspicuous(unnoticeable) even if the outside sheathing has a conventional platethickness.

In this embodiment, a “floating framework structure” is employed inwhich continuous outside sheathing reinforcement members (horizontalframework members) are laser-welded to the outside sheathing and mainframes (vertical framework members) provided on the outside sheathingreinforcement members so as to extend perpendicularly thereto.

In joining the horizontal framework members and the vertical frameworkmembers by laser welding so as to make the two types of frameworkmembers to cross each other, the above-described hold-down roller isused to ensure intimate contact between lap portions during joining. Ifthe flange of each vertical framework member positioned on the upperside is insufficient in plate thickness or stiffness, the flange part ofthe vertical frame member becomes deflected along with the horizontalframework members, so that intimate contact therebetween cannot beensured. An additional large-sized hold-down jig is needed to avoid sucha problem, which will considerably worsen the productivity and the costefficiency.

If such a remedy is taken as to make the plate thickness (2.5 mm) ofeach vertical framework member positioned on the upper side larger thanthat (1 mm) of each horizontal framework member or to fold a flange endedge portion of each vertical framework member so as to impart thevertical framework member with sufficient stiffness, the horizontalframework members can be prevented from being deflected and, hence, lapportions to be joined together can be welded with use of a simplehold-down roller. This feature is excellent in productivity and costefficiency.

In cases where a crossing part of horizontal and vertical frameworkmembers is located adjacent a corner of an opening of the outsidesheathing such as a side window or a side doorway, a large shearingforce is likely to work between the horizontal and vertical frameworkmembers and, hence, a sufficient strength cannot be ensured only by thelap joint between the horizontal framework member and a narrow flangepart of the vertical framework member, as the case may be.

To avoid such an inconvenience, a flange part 341 a of verticalframework member 341 may be widened to form a gusset-shaped extension(hereinafter will be referred to as gusset part) 341 b, thereby ensuringa sufficient joint area. In this case, the associated horizontalframework members are preferably extended to a peripheral edge so as tomatch the corner shape of the opening of the outside sheathing 306.

A three-dimensional gusset can be formed by joining of theaforementioned gusset part 341 b to the extensions of the outsidesheathing reinforcement members 307. The gusset part 314 b may be acomponent separate from the vertical framework member 341 positioned onthe upper side. It is desirable that the gusset part 314 b and theextensions of the outside sheathing reinforcement members 307 be weldedtogether by laser welding.

Such a feature makes it possible to relieve the shear stress produced onthe lap portions of the horizontal framework member (reinforcementmember 307) and vertical framework member 341. Further, thethree-dimensional gusset can relieve the stress produced on the outsidesheathing 306.

If laser welding is used for narrow and small portions such as lapportions of the horizontal framework members (outside sheathingreinforcement members 307) and the vertical framework members 341 andlap portions of the horizontal framework members (outside sheathingreinforcement members 307) and the gusset part 314 b, discrete shortweld lines result. For this reason, such a narrow and small portion maybe subjected to such laser welding as to form a closed ring-shaped weldline U1 by rotation of the optical axis of laser beam (ring welding toform a weld line ring having a diameter of about 10 to 20 mm), as shownin FIG. 33(a). The ring diameter is appropriately determined from ajoint area, required strength and the like.

By so doing, a relatively long weld line U can be ensured even in anarrow and small portion and, hence, a required strength can beobtained. In addition, fissuring can hardly occur because there is noweld end.

If a tubular hold-down jig 351 is used in ring welding as shown in FIG.33(b), intimate contact between the upper and lower members can beensured to allow proper laser welding to be achieved.

FIG. 34(a) shows one example of a structure for mounting interior trimor equipment to a side outside sheathing. As shown in FIG. 34(a), acontinuous rail member 361, which is formed into a rail shape by foldinga plate material, is mounted on the side outside sheathing 306 to extendin the longitudinal direction of the railway car or in the directionperpendicular thereto. This rail member 361 has a recess for engagementwith the head of a fitting member 362 such as a square-headed specialbolt or nut. The rail member 316 is formed by folding a single platematerial in such a manner as to form double-wall portions on oppositesides of the recess and fitting portions extending from opposite endportions of the respective outer walls of the double-wall portions inopposite directions. A seat or an interior panel as the interior trim orequipment is attached to the fitting member 362 engaged with the railmember 361. In this way, the interior trim or equipment can be mountedon the side outside sheathing.

It is possible that a rail member 364 is mounted on a horizontalframework member (carline) 363 of the roof construction so as to extendin the longitudinal direction of the railway car and engaged with afitting member 365, as shown in FIG. 34(b). Such a structure allows aunit of ceiling air duct, fluorescent lamp or the like as the interiortrim or equipment to be suspended from the horizontal framework member363 of the roof construction.

It is also possible that a rail member 371 is joined between floorboards (steel decks) of the underframe 305 so as to extend in thelongitudinal direction of the railway car for seats to be mountedthereon (seat track), as shown in FIG. 35(a).

A rail member 372 may be used as a cross beam of the underframe as shownin FIG. 35(b) so as to allow underfloor equipment to be suspendedtherefrom.

In this case, the rail member is formed to have a rail shape by drawingor roll diffusion bonding. This continuous rail member is joined to theoutside sheathing or the main frame by laser welding. Though laserwelding is used to join the rail member to the main frame in thisembodiment, arc welding or resistance spot welding may be used instead.

By so doing, it is possible to reduce the parts count and the man-hourfor mounting secondary structural members such as internal framework andfacilitate dimensional control.

The aforementioned drawing is capable of forming the rail member farmore inexpensively and accurately than the common press forming. As aresult, precise fitting of the head of a fitting member into the railcan be realized.

The subsequent description will be made of a method of assembling a sideconstruction and a method including preparing plural divided units toform a side construction and then joining these units together by laserwelding.

(1) According to a first method, two types of side units 381A and 381B,a frieze board unit 382 and a door mask 383 are manufactured separatelyand then joined together to form a side construction 384 by laserwelding, as shown in FIGS. 36(a) and 36(b). In this case, the side units381A and 381B and the frieze board unit 382 may be assembled by laserwelding.

(1) According to a second method, side units 381C and 381D includingrespective frieze board parts 381Ca and 381Da and the door mask 383 aremanufactured separately and then joined together to form a sideconstruction 385 by laser welding, as shown in FIGS. 37(a) and 37(b).

The aforementioned side units 381C and 381D are assembled by laserwelding. The door mask 383 is also joined by laser welding. The friezeboard parts 381Ca and 381Da of the side unit 381C and 381D are joined toeach other by laser welding or arc welding.

(3) According to a third method, side units 381C and 381C includingrespective frieze board parts 381Ca and 381Da, and a door mask 383Aextending to cover the frieze board part are manufactured separately andthen joined together to form a side construction 386 by laser welding,as shown in FIGS. 38(a) and 38(b). The extension of the door mask 383Acovering the frieze board part is located exteriorly of the frieze boardparts 381Ca and 381Da of the respective side units 381C and 381D.

The side units 381C and 381D are assembled by laser welding. The methodof joining the door mask 383A and the method of joining the frieze boardparts 381Ca and 381Da of the respective side units 381C and 381D are thesame as described above.

(4) According to a fourth method, side units 381E and 381F includingrespective door mask mating halves 381Ea and 381Fa and respective friezeboard parts 381Ea and 381Fa are manufactured separately and then joinedtogether to form a side construction 387 by laser welding, as shown inFIGS. 39(a) and 39(b).

The side units 381E and 381F are assembled by laser welding.

These methods are advantageous in enhancing the speed at which each ofthe side constructions 384 to 387 is constructed, improving the sideoutside sheathing in aesthetic terms, rendering each side constructionsealless with its watertightness ensured, and improving the dimensionalaccuracy.

According to a method of assembling each of the aforementioned sideunits 381A to 381F, wainscot sheathing 141A, pier plate 141B and friezeboard 141 c are divisions of the side outside sheathing, and theirrespective outside sheathings 141A to 141C are joined with respectiveoutside sheathing reinforcement members 142A to 142C by laser welding toform wainscot panel 143A, pier panel 143B and frieze board panel 143C,as shown in FIG. 40 for example.

In this case, the outside sheathing 141A and the flange part of theoutside sheathing reinforcement member 142A form a lap joint, andpartial penetration laser welding is conducted from the outsidesheathing reinforcement member 142A side to joint the two together, asshown in FIGS. 41(a) to 41(d). This holds true for the joint between theoutside sheathing 141B and the outside sheathing reinforcement member142B and the joint between the outside sheathing 141C and the outsidesheathing reinforcement member 142C. Note that a window frame (notshown) is previously joined to the pier plate 141B by laser welding in asimilar manner.

The pier plate 141B and the frieze board 141C having respective lowerend edges each formed with a joggled portion. Lap joints are formedbetween the joggled portion of the lower end edge of the pier plate 141Band the upper end of the wainscot sheathing 141A and between the joggledportion of the lower end edge of the frieze board 141C and the upper endof the pier plate 141B by laser welding to form a side unit 121 (seeFIG. 42). Note that reference numerals 122 and 123 denote a windowopening portion and a doorway opening portion, respectively.

Further, belt rail 145 and window heads 144A and 144B, which are locatedat the respective lap joints, are joined to the outside sheathing bylaser welding (see FIG. 43). The belt rail 145 and window heads 144A and144B may be previously joined to the pier plate 141B if these memberscan be positioned so as not to hinder the laser welding between thewainscot panel 143A and the pier panel 143B or between the pier paneland the frieze board panel.

Subsequently, the vertical posts 146 including front and rear door endposts 146A and 146B and intermediate post 146C are joined to the top ofeach of the outside sheathing reinforcement members 142A by laserwelding (see FIGS. 40, 41(d) and 43). The weld line resulting from thelaser welding may be either a short weld line or a ring-shaped weld lineensuring further stabilized joint strength. Reference numeral 148denotes a doorframe.

Finally, gussets 150A to 150D are joined to respective portions eachlocated on and around each of the joints between the door end post 146Band the belt rail 145 and between the door end post 146B and the windowheads 144A and 144B by laser welding, thus completing the side unit 121(see FIG. 44).

The side unit manufacturing method described above is applicable to anyone of the end construction unit, roof construction unit and underframeunit.

In the case of the roof construction unit, roof outside sheathings arejoined to each other by continuous laser welding and then carlines arejoined to the roof outside sheathings by laser welding from the outsidesheathing side. Since the roof construction does not call for a highaesthetic value, the laser welding for the roof construction may beeither of piecing welding and partial penetration welding.

In the case of the underframe unit, side beams and cross beams arejoined together using gussets by laser welding or resistance spotwelding to form framework and then a floor board comprising steel decksis joined to the cross beams by laser welding from the floor board side.Since the lower part of the underframe and the floor board surfacebecome blind spots or hidden portions which do not call for anyaesthetic value when the railway car is completed, the laser weldingused therefor may be either of piecing welding and partial penetrationwelding.

Finally, the body structure for rolling stock is assembled by joiningthe right and left side units, roof unit, underframe unit and front andrear end units together according to the methods illustrated in FIGS. 25to 37.

It should be noted that the foregoing embodiments are merelyillustrative and may be modified variously without departing from thespirit of the present invention and, hence, the present invention is notlimited to the foregoing embodiments.

1. A laser welding method comprising the steps of: lapping plural platemembers one over another; continuously irradiating one of the lappedplate members with a moving laser beam from an antiplane direction toheat said lapped plate members until a region extending to an innerportion of another one of said lapped plate members which is positionedon an opposite side away from the plate member irradiated with the laserbeam becomes molten; and joining said plural plate members togetherwhile controlling the laser power or moving speed of the laser beam soas not to allow a bottom portion of a resulting molten pool to reach anoutside surface of said plate member positioned on the opposite side,the laser welding method characterized by previously polishing theoutside surface of said opposite-side plate member substantially inparallel with the direction of a weld line to be formed by the laserbeam.
 2. A laser-welded joint comprising plural plate members joinedtogether by: lapping said plural plate members one over another;continuously irradiating one of the lapped plate members with a movinglaser beam from an antiplane direction to heat said lapped plate membersuntil a region extending to an inner portion of another one of saidlapped plate members which is positioned on an opposite side away fromthe plate member irradiated with the laser beam becomes molten; andjoining said plural plate members together while controlling the laserpower or moving speed of the laser beam so as not to allow a bottomportion of a resulting molten pool to reach an outside surface of theplate member positioned on the opposite side, the laser-welded jointcharacterized in that the outside surface of said opposite-side platemember is polished substantially in parallel with the direction of aweld line formed by the laser beam.
 3. An outside sheathing panelcomprising an outside sheathing, and a first reinforcement member lappedover said outside sheathing interiorly of said outside sheathing, saidoutside sheathing and said first reinforcement member being joinedtogether by being continuously irradiated with a moving laser beam fromthe first reinforcement member side so as to be heated until a regionextending to an inner portion of said outside sheathing becomes moltenwith the laser power or moving speed of the laser beam controlled so asnot to allow a bottom portion of a resulting molten pool to reach anoutside surface of said outside sheathing, the outside sheathing panelcharacterized in that the outside surface of said outside sheathing ispolished substantially in parallel with the direction of a weld lineformed by the laser beam.
 4. A body structure for rolling stockcomprising an outside sheathing panel as a side outside sheathing of aside construction, said outside sheathing panel comprising an outsidesheathing, and a first reinforcement member lapped over said outsidesheathing interiorly of said outside sheathing, said outside sheathingand said first reinforcement member being joined together by beingcontinuously irradiated with a moving laser beam from the firstreinforcement member side so as to be heated until a region extending toan inner portion of said outside sheathing becomes molten with the laserpower or moving speed of the laser beam controlled so as not to allow abottom portion of a resulting molten pool to reach an outside surface ofsaid outside sheathing, the body structure characterized in that outsidesurface of said outside sheathing of said outside sheathing panel ispolished substantially in parallel with the direction of a weld lineformed by the laser beam; and the direction of said weld line coincideswith a specific direction.
 5. The body structure for rolling stockaccording to claim 4, wherein: said first reinforcement member has aportion shaped like a hat in section; and said portion has a body partsubstantially U-shaped in section and flange parts extendingcontinuously from opening defining edges of said body part in oppositedirections and joined to said outside sheathing by laser welding, saidbody part extending in a longitudinal direction of a railway car.
 6. Thebody structure for rolling stock according to claim 5, wherein: thesubstantially U-shaped section of said body part defines therein a spacegradually widening as said space extends toward the open side; saidfirst reinforcement member is provided on the opposite side away fromsaid outside sheathing with a second reinforcement member which isshaped like a hat in section and extends in a direction perpendicular tosaid first reinforcement member; and said second reinforcement memberhas flange portions joined to said body part of said first reinforcementmember.
 7. A body structure for rolling stock comprising an outsidesheathing of a side construction, and third reinforcement members joinedto said outside sheathing interiorly of said outside sheathing by laserwelding for stiffening said outside sheathing, a part or all of saidthird reinforcement members being arranged in a longitudinal directionof a railway car, the body structure characterized in that said thirdreinforcement members are arranged at a pitch defined by spacing betweenadjacent weld lines formed by said laser welding, said pitch beingestablished such that uniaxial stresses σx and σy in respective of thelongitudinal direction of the railway car and a direction perpendicularthereto and a shear stress τ, which are obtained by resolution of anin-plane stress component exerted on said outside sheathing, are notlarger than a buckling stress value determined by Euler's bucklingformula and a plasticity correction formula.
 8. A body structure forrolling stock comprising an outside sheathing of a side construction,and third reinforcement members joined to said outside sheathinginteriorly of said outside sheathing by laser welding for stiffeningsaid outside sheathing, a part or all of said third reinforcementmembers being arranged in a longitudinal direction of a railway car, thebody structure characterized in that said third reinforcement membersare arranged at a pitch defined by spacing between adjacent weld linesformed by said laser welding, said pitch being established based on abuckling strength of a portion at which a load to be imposed on saidthird reinforcement members becomes maximized in a direction in whichsaid third reinforcement members extend.
 9. A body structure for rollingstock comprising an outside sheathing of a side construction, and thirdreinforcement members joined to said outside sheathing interiorly ofsaid outside sheathing by laser welding for stiffening said outsidesheathing, said third reinforcement members being arranged in a raildirection, the body structure characterized in that: said thirdreinforcement members are each shaped like a hat in section and eachhave a U-shaped part substantially U-shaped in section and two fittingflange parts extending continuously from end edges of the U-shaped partin opposite directions and joined to said outside sheathing by laserwelding; and said U-shaped part of each of said third reinforcementmembers is smaller in width than a U-shaped part of a first memberprovided on another part of the body structure.
 10. The body structurefor rolling stock according to claim 7, wherein: said thirdreinforcement members are each shaped like a hat in section and eachhave a U-shaped part substantially U-shaped in section and two fittingflange parts extending continuously from end edges of said U-shaped partin opposite directions and joined to said outside sheathing by laserwelding; and a fourth reinforcement member is previously mounted in saidU-shaped part of each of said third reinforcement members for enhancingan opening portion of said U-shaped part.
 11. The body structure forrolling stock according to claim 8, wherein: said third reinforcementmembers are each shaped like a hat in section and each have a U-shapedpart substantially U-shaped in section and two fitting flange partsextending continuously from end edges of said U-shaped part in oppositedirections and joined to said outside sheathing by laser welding; and afourth reinforcement member is previously mounted in said U-shaped partof each of said third reinforcement members for enhancing an openingportion of said U-shaped part.
 12. The body structure for rolling stockaccording to claim 9, wherein: said third reinforcement members are eachshaped like a hat in section and each have a U-shaped part substantiallyU-shaped in section and two fitting flange parts extending continuouslyfrom end edges of said U-shaped part in opposite directions and joinedto said outside sheathing by laser welding; and a fourth reinforcementmember is previously mounted in said U-shaped part of each of said thirdreinforcement members for enhancing an opening portion of said U-shapedpart.
 13. The body structure for rolling stock according to claim 10,wherein said U-shaped part of each of said third reinforcement membershas a head portion having an opening or a notch for allowing said fourthreinforcement member to be joined to said outside sheathing by laserwelding.
 14. The body structure for rolling stock according to claim 11,wherein said U-shaped part of each of said third reinforcement membershas a head portion having an opening or a notch for allowing said fourthreinforcement member to be joined to said outside sheathing by laserwelding.
 15. The body structure for rolling stock according to claim 12,wherein said U-shaped part of each of said third reinforcement membershas a head portion having an opening or a notch for allowing said fourthreinforcement member to be joined to said outside sheathing by laserwelding.
 16. The body structure for rolling stock according to claim 7,wherein each of the third reinforcement members has a root portionjoined to said outside sheathing by laser welding, and a hollow bulgedportion having closed ends.
 17. The body structure for rolling stockaccording to claim 8, wherein each of the third reinforcement membershas a root portion joined to said outside sheathing by laser welding,and a hollow bulged portion having closed ends.
 18. The body structurefor rolling stock according to claim 7, wherein an inside plate frame isjoined to end portions of those third reinforcement members locatedadjacent a corner of an opening portion formed in said outside sheathingby laser welding to interconnect said end portions.
 19. The bodystructure for rolling stock according to claim 8, wherein an insideplate frame is joined to end portions of those third reinforcementmembers located adjacent a corner of an opening portion formed in saidoutside sheathing by laser welding to interconnect said end portions.20. The body structure for rolling stock according to claim 9, whereinan inside plate frame is joined to end portions of those thirdreinforcement members located adjacent a corner of an opening portionformed in said outside sheathing by laser welding to interconnect saidend portions.
 21. The body structure for rolling stock according toclaim 18, wherein said third reinforcement members and said inside plateframe are joined together by laser welding so as to form closed weldlines.
 22. The body structure for rolling stock according to claim 19,wherein said third reinforcement members and said inside plate frame arejoined together by laser welding so as to form closed weld lines. 23.The body structure for rolling stock according to claim 20, wherein saidthird reinforcement members and said inside plate frame are joinedtogether by laser welding so as to form closed weld lines.
 24. The bodystructure for rolling stock according to claim 7, wherein: said outsidesheathing has an opening portion having a peripheral edge provided withan edging member; and said edging member is formed with a notch forallowing a hold-down roller to advance thereinto in laser-welding saidthird reinforcement members to said outside sheathing, said hold-downroller being configured to press a portion of each of said thirdreinforcement members to be joined against said outside sheathing whilemoving together with the laser beam.
 25. The body structure for rollingstock according to claim 8, wherein: said outside sheathing has anopening portion having a peripheral edge provided with an edging member;and said edging member is formed with a notch for allowing a hold-downroller to advance thereinto in laser-welding said third reinforcementmembers to said outside sheathing, said hold-down roller beingconfigured to press a portion of each of said third reinforcementmembers to be joined against said outside sheathing while movingtogether with the laser beam.
 26. The body structure for rolling stockaccording to claim 9, wherein: said outside sheathing has an openingportion having a peripheral edge provided with an edging member; andsaid edging member is formed with a notch for allowing a hold-downroller to advance thereinto in laser-welding said third reinforcementmembers to said outside sheathing, said hold-down roller beingconfigured to press a portion of each of said third reinforcementmembers to be joined against said outside sheathing while movingtogether with the laser beam.
 27. The body structure for rolling stockaccording to claim 7, wherein: said third reinforcement members are eachshaped like a hat in section and each have a U-shaped part substantiallyU-shaped in section and two fitting flange parts extending continuouslyfrom end edges of said U-shaped part in opposite directions and joinedto said outside sheathing by laser welding; an inside plate frame isjoined to free edge ends of said third reinforcement members outwardlyof said free edge ends; and said inside plate frame has a body partjoined to said U-shaped parts of said third reinforcement members, a legpart extending continuously from said body part in such a direction asto close said free edge ends, and a flange part extending continuouslyfrom said leg part and joined to said outside sheathing.
 28. The bodystructure for rolling stock according to claim 8, wherein: said thirdreinforcement members are each shaped like a hat in section and each hasa U-shaped part substantially U-shaped in section and two fitting flangeparts extending continuously from end edges of said U-shaped part inopposite directions and joined to said outside sheathing by laserwelding; an inside plate frame is joined to free edge ends of said thirdreinforcement members outwardly of said free edge ends; and said insideplate frame has a body part joined to said U-shaped parts of said thirdreinforcement members, a leg part extending continuously from said bodypart in such a direction as to close said free edge ends, and a flangepart extending continuously from said leg part and joined to saidoutside sheathing.
 29. The body structure for rolling stock according toclaim 7, wherein: an inside plate frame is joined to free edge ends ofsaid third reinforcement members outwardly of said free edge ends; andsaid inside plate frame has a first flange part joined to said U-shapedparts of said third reinforcement members, a second flange part joinedto said outside sheathing, first and second leg parts extending parallelfrom respective of said first and second flange parts to a same height,and a horizontal part extending parallel with said outside sheathing tointerconnect upper end edges of respective of said first and second legparts.
 30. The body structure for rolling stock according to claim 8,wherein: an inside plate frame is joined to free edge ends of said thirdreinforcement members outwardly of said free edge ends; and said insideplate frame has a first flange part joined to said U-shaped parts ofsaid third reinforcement members, a second flange part joined to saidoutside sheathing, first and second leg parts extending parallel fromrespective of said first and second flange parts to a same height, and ahorizontal part extending parallel with said outside sheathing tointerconnect upper end edges of respective of said first and second legparts.
 31. The body structure for rolling stock according to claim 7,wherein at least one of a side outside sheathing of said sideconstruction and a roof outside sheathing of a roof construction has anend edge portion extending in the longitudinal direction of said railwaycar and having a folded part by which said side outside sheathing andsaid roof outside sheathing are joined together by continuous laserwelding.
 32. The body structure for rolling stock according to claim 8,wherein at least one of a side outside sheathing of said sideconstruction and a roof outside sheathing of a roof construction has anend edge portion extending in the longitudinal direction of said railwaycar and having a folded part by which said side outside sheathing andsaid roof outside sheathing are joined together by continuous laserwelding.
 33. The body structure for rolling stock according to claim 9,wherein at least one of a side outside sheathing of said sideconstruction and a roof outside sheathing of a roof construction has anend edge portion extending in the longitudinal direction of said railwaycar and having a folded part by which said side outside sheathing andsaid roof outside sheathing are joined together by continuous laserwelding.
 34. The body structure for rolling stock according to claim 31,wherein said folded part has a U-shaped section which opens upwardly ofsaid roof outside sheathing.
 35. The body structure for rolling stockaccording to claim 32, wherein said folded part has a U-shaped sectionwhich opens upwardly of said roof outside sheathing.
 36. The bodystructure for rolling stock according to claim 33, wherein said foldedpart has a U-shaped section which opens upwardly of said roof outsidesheathing.
 37. The body structure for rolling stock according to claim7, wherein: an end outside sheathing of an end construction and a sideoutside sheathing of said side construction are joined togetherperpendicularly to each other via an end corner post having asubstantially L-shaped section; said end corner post is either joinedwith a separate member defining a closed space cooperatively with saidend corner post by continuous laser welding or formed with a closedspace forming part defining the closed space cooperatively with said endcorner post; and said closed space is configured to function as arainwater guttering for guiding rainwater from a roof to the ground. 38.The body structure for rolling stock according to claim 8, wherein: anend outside sheathing of an end construction and a side outsidesheathing of said side construction are joined together perpendicularlyto each other via an end corner post having a substantially L-shapedsection; said end corner post is either joined with a separate memberdefining a closed space cooperatively with said end corner post bycontinuous laser welding or formed with a closed space forming partdefining the closed space cooperatively with said end corner post; andsaid closed space is configured to function as a rainwater guttering forguiding rainwater from a roof to the ground.
 39. The body structure forrolling stock according to claim 9, wherein: an end outside sheathing ofan end construction and a side outside sheathing of said sideconstruction are joined together perpendicularly to each other via anend corner post having a substantially L-shaped section; said end cornerpost is either joined with a separate member defining a closed spacecooperatively with said end corner post by continuous laser welding orformed with a closed space forming part defining the closed spacecooperatively with said end corner post; and said closed space isconfigured to function as a rainwater guttering for guiding rainwaterfrom a roof to the ground.
 40. The body structure for rolling stockaccording to claim 7, wherein said outside sheathing has an outsidesurface which is surface-finished by brushing or shot blasting.
 41. Thebody structure for rolling stock according to claim 8, wherein saidoutside sheathing has an outside surface which is surface-finished bybrushing or shot blasting.
 42. The body structure for rolling stockaccording to claim 9, wherein said outside sheathing has an outsidesurface which is surface-finished by brushing or shot blasting.
 43. Thebody structure for rolling stock according to claim 7, wherein saidoutside sheathing is interiorly fitted with a continuous rail memberwhich is U-shaped in section for mounting interior trim or equipment onsaid outside sheathing by means of a mounting bolt having a head portionconfigured to engage said rail member.
 44. The body structure forrolling stock according to claim 8, wherein said outside sheathing isinteriorly fitted with a continuous rail member which is U-shaped insection for mounting interior trim or equipment on said outsidesheathing by means of a mounting bolt having a head portion configuredto engage said rail member.
 45. The body structure for rolling stockaccording to claim 9, wherein said outside sheathing is interiorlyfitted with a continuous rail member which is U-shaped in section formounting interior trim or equipment on said outside sheathing by meansof a mounting bolt having a head portion configured to engage said railmember.
 46. The body structure for rolling stock according to claim 7,wherein said outside sheathing is thicker than each of said thirdreinforcement members and has a plate thickness of not less than 3 mm.47. The body structure for rolling stock according to claim 8, whereinsaid outside sheathing is thicker than each of said third reinforcementmembers and has a plate thickness of not less than 3 mm.
 48. The bodystructure for rolling stock according to claim 9, wherein said outsidesheathing is thicker than each of said third reinforcement members andhas a plate thickness of not less than 3 mm.
 49. The body structure forrolling stock according to claim 7, wherein: said outside sheathingdefines an opening portion having a peripheral edge exteriorly providedwith an edging member; and said edging member has a joint portion joinedto said outside sheathing by laser welding from the outside sheathingside, said joint portion comprising a folded plate structure formed byfolding a flat plate.
 50. The body structure for rolling stock accordingto claim 8, wherein: said outside sheathing defines an opening portionhaving a peripheral edge exteriorly provided with an edging member; andsaid edging member has a joint portion joined to said outside sheathingby laser welding from the outside sheathing side, said joint portioncomprising a folded plate structure formed by folding a flat plate. 51.The body structure for rolling stock according to claim 9, wherein: saidoutside sheathing defines an opening portion having a peripheral edgeexteriorly provided with an edging member; and said edging member has ajoint portion joined to said outside sheathing by laser welding from theoutside sheathing side, said joint portion comprising a folded platestructure formed by folding a flat plate.